The max supported number of SATA devices (using cable adapters) on a Dell SAS 6/iR adapter

This has been covered here... See the related links on the right pane of this question.

Right now, the market conditions are such that you should try to use SAS disks everywhere you can.

• Enterprise SAS disks are your fastest and most resilient rotating media available at 10,000 and 15,000 RPM. Performance-optimized
• Nearline or Midline SAS are usually mechanically-equivalent to 7,200 RPM SATA disks, but feature a SAS interface and offer the benefits of the SAS protocol. They are available in higher capacities than enterprise SAS disks. They have a slight price premium over the same sized SATA drives. Capacity-optimized
• Solid-state disks (SSDs) are a higher tier of storage and shouldn't be compared directly with rotating media. Their main characteristic is higher random read and write performance, but the details are beyond the scope of this question.

Also see:

SAS or SATA for 3 TB drives?

How can a single disk in a hardware SATA RAID-10 array bring the entire array to a screeching halt?

This is actually a complicated electrical question dealing with capacitance, crosstalk, signal loss, and a number of other factors. That stuff makes my brain hurt though, so as a rule of thumb let's just say that passive cables should be limited to the shortest length, and should always be kept under the specification limits.

As a practical matter a cable slightly longer than the specification limits will probably still work, but it would be Bad And Wrong of you to attempt such a thing, and the engineer who wrote the specification will find you and kill you in your sleep.

So for a configuration of:
[Controller]<--SAS Cable--><--SATA Cable-->[SATA Drive]
or
[Controller]<--SAS-TO-SATA Cable-->[SATA Drive]
the two total cable length between the controller and drive should be limited to a maximum length of 1 meter (the SATA limit).
One meter is a REALLY FREAKN' LONG CABLE -- I would be hard pressed to find a cable routing that would exceed that in most servers/racks. If you're in that situation with external cabling in your rack the usual suggestion is to move a machine to reduce the distance.

When active components are involved (as they would be in your case) things are a little different.
The length of any passive cable is still limited, but active components (at least partially) reset the distance counter, so in a configuration like this:
[Controller]<--ESATA Cable-->[JBOD SAS Backplane]<==SAS Cables==>[SAS Disks]
the ESATA cable length is limited to 2 meters (the ESATA specification limit), and the cables betwee the JBOD's backplane and the SAS drives is limited to whatever the SAS limit is (I think it's 3 meters but don't quote me on that).

As a practical matter you can't just keep cabling things together for an infinitely long chain (even if you're sticking an active component in at each distance limit) - eventually the delays introduced by the cabling and the hardware would start to cause problems. Whether you hit other practical limits before that (like controller bandwidth saturation).