These strips are usually arranged in numerous parallel segments of a few series LEDs. The RGB strips can be a little different, but they work basically the same way. It looks like your strip is analog control and not digital (which would have little ICs in the strip to control individual LED color), so the specific color ground lines are all in parallel. You should be able to connect numerous strips in parallel - 12V to 12V, red to red, green to green, and blue to blue to blue. That is the same reason you can cut the strips into smaller segments if you want.
There is a limit to how much current can flow through the strip itself, which varies between manufacturers. Your other limit to how many strips can be connected together is how much power your supply / remote controller can handle. It should specify a limit in watts / current or total number of LEDs or parallel segments. That information should be on a sticker on the power supply or be listed in the manual.
From your product page:
Power 72 W
Which is 6A at 12V DC. That means your power supply/controller has to be able to handle at least 144 W (12A @ 12V DC) to connect two strips in parallel. Although, it is never a good idea to run something long term near its maximum ratings, so the supply should really be rated for something closer to 200 W.
If you are able to power multiple strips from your supply, it would be best to power them with a star topology so the current for each strip is separated. For example, connecting Christmas lights together end to end is daisy chaining, and the current for every light strand has to flow through the strands before it. Powering multiple light strands from one power strip is more of a star topology, with the current to each strip only flowing through itself (think of the power strip as your power supply).
WS2801's take 24 bits from the data stream and passes the rest to its output. In theory you could control a very large number of LEDs, but in practice, you are limited by data integrity, transmission speed, etc.
In my experience, controlling 170 LED's is common with these kinds of strips, because that works out to be 510 8-bit values, which fits within a DMX-512 "Universe" (which supports 512 such 8-bit values).
I have successfully controlled 300 LEDs in a strip of WS2811's (a 5-meter length with 60 LEDs per meter, similar to WS2801 except they aren't pure SPI, and the IC is actually inside the 5050 LED package) using a custom ARM-based DMX-to-WS2811 decoder. However, I don't recommend it because of latency. In my application, I needed to change all LEDs at the same rate as a video source, about 30 times per second.
With 32 LEDs/meter, you should be able to run approximately 5 meters on a given output, if you limit it to < 170 LEDs.
Voltage Drop
The voltage drop on a 5V LED strip (especially with the 0.5 oz. copper they are usually made from) can be significant. If you only provide power at one end of a 5 meter strip, LEDs driven white will not appear uniform. LEDs at the power-end of the strip will appear white, while LEDs further away will start to appear tinted pink or cyan. The manufacturer recommends providing power at both ends of the strip, but even then there is a gradient. Typically these LEDs require ~60mA at full white, so a 5 meter strip (160 LEDs) will require 9.6A, which is quite a bit when you consider that the power leads are often only 16 or 18 AWG. I recommend providing power at 2- or 3-meter intervals if possible, but it depends on your application.
Raspberry Pi: SPI Ports
According to the Raspberry Pi Education Manual (pg 130), the device has "five pins available to connect devices to the Raspberry Pi using SPI." One site indicates that:
The Raspberry Pi only implements master mode at this time and has 2 chip-select pins, so can control 2 SPI devices. (Although some devices have their own sub-addressing scheme so you can put more of them on the same bus)
Of course, you could probably "bit bang" your own SPI using other I/O pins, but I'm not familiar with the Raspberry Pi so I can't say what the performance would be like. Ultimately, you could control many lines using a single Raspberry Pi, but the speed at which you refresh or change the LED colors is subject to change.
Unfortunately I am not experienced with SPI extenders, so I cannot advise on the use of them. Perhaps someone else can address that part of your question.
Edit:
DMX
Per your edit: For slower refreshes like 5 Hz, it gets much easier (compared to maintaining 30 frames per second to match a video source). You might want to consider using a differential signal (like DMX) instead of SPI, which would allow you to control strips that are physically distant from a controller. SPI is quite limited on distance; if you think about it, the WS2801's act like repeaters, ensuring signal integrity down the line. If you want all LED strips to have the same color and brightness, then you don't need to treat a second strip differently from the first: you can send both strips the same information. Using DMX simplifies things because of its flexibility and use in the lighting industry.
For example, you would run say 5 meters at a time, giving each a DMX-to-SPI converter. You could then have each strip on its own universe if you still needed to individually control LEDs. Alternatively, you can have them all on the same universe, and they will all respond together. Thus, you could have a gradient from say blue to red on strip #1, and all strips would repeat the pattern. You can make your own DMX-to-SPI converters or get ready-made ones (for example the inexpensive "miniDMX 2801" available on eBay). I would recommend investigating how you could send DMX data using a Raspberry Pi.
If you assign each strip a different universe, you can then send unique data to each one. You'll have to calculate whether you have time to send all of the information needed in 1/5 s.
SPI
When you send data using SPI, the WS2801 waits for a 500μs or longer low level on the CLK. Once that happens, it latches the first 24 bits and then relays anything else. I am not sure how many of these in sequence you can actually connect. In theory, it would be however many bits you can send in 1/5 second (before the next frame is needed). However I wouldn't realistically expect this to work well for long runs (>500), but I could be wrong.
Power
Regardless of how you send data, you still need to provide power at regular intervals such that you are satisfied there are no color inconsistencies and you're not overloading any wires.
Best Answer
The datasheet for the WS2811 is where you need to look.
It is a constant-current three-channel driver that operates on 4.5 ~ 5.5 V, with absolute maximum ratings of 6 ~ 7 V. The output voltage is specified as 12 V maximum. Output current is maintained at 18.5 mA on each of the three output channels.
Therefore, the maximum number of LEDs you can drive with one output is dependent on the voltage drop of the LEDs. The total voltage drop across all LEDs would need to be less than the max output voltage.
Consider page 5 of the datasheet:
Here, using 5V, only one LED is being used per channel.
In this example, three LEDs are being driven per channel at 12V.
Simply put, you could drive as many LEDs as you like, provided you don't exceed 12V or require more than 18.5 mA. LEDs in series will sum forward voltages, while LEDs in parallel will share current.
Say you found an LED with a 2.4 \$V_F\$ and was satisfactorily bright at 10 mA. In theory you could connect ten of them in series-parallel (two parallel sets of five in series). The total \$V_F\$ would be 12, and each series set would receive half the current (9.25 mA).
To drive LEDs that in total exceed this driver's capability, would require additional components like transistors. Depending on your intended application, it might be more appropriate to use additional drivers and duplicate the data line to them. I have successfully driven four lines of drivers at once with the same data source, but there are a number of factors to take into consideration when doing this (slew rate, distance, etc.).