Electronic – the power voltage pin outs for a Playstation 5 PS5 power drive connector on the motherboard

As Passerby says, the main reason why there are several wires for any given voltage is that a computer may require tens of amps, so one wire by itself isn't enough.

The reason for all the voltages is largely historical. For many years, the standard voltage for digital logic circuits was 5V. Many circuits still run at 5V.

However, later chip designs could be made to work at lower voltages, and would run cooler when powered at a lower voltage. So 3.3V was added to the power supply.

+12V is mainly used for powering motorized things, like disk drives and fans. It's also needed for RS232 serial ports. RS232 also requires -12V, so power supplies often have a low-current -12V line as well.

The 5Vsb line is a "standby" power line, to keep some parts of the computer powered on while the computer is in "sleep" mode.

As @bitshift says, you'll likely need to get the actual spec to be sure.

1.5A may well be a real supply requirement, though. The cables used are actually rated to 4.5A per supply rail. The connectors contain 3.3V, 5V, and 12V rails.

A regular (magnetic 3.5") hard drive can dissipate almost 10W. This energy is probably split across the three available power rails. Since I have no idea what the split may be, the worst case numbers look something like this :

All 10W @ 3.3V - 3.33A
All 10W @ 5V   - 2A
All 10W @ 12V  - 0.83A

The actual spec could help you understand how the load is allowed to be spread over the three supplies and what the effective ratings needed are.

If you don't care about it being SATA compliant, you could assume the load is lower by a factor of 2 or so since you're only using SSDs.