1.Are you doing a 2nd fermentation to carbonate the beer or forced carbonation?
2.Or are you referring to how much pressure to transfer between fermenter and keg?
1.
If fermenting then follow the instructions on the packet for how much sugar to add to the keg(s).
If force carbonating then look up the CO2 volume recommended for a stout (probably about 2.0) and the temperature you will be storing at to get the pressure needed for the CO2 volume. Then leave for about a week (you can always test it before hand!).
https://drhansbrewery.com/beercarbonationcalculator/
There are probably quite a few calculators but this one is simple so l get on with it.
2.
The lowest amount you can get away with. I use about 5psi for non pressurised ferments. Then you don't need a spunding valve on the keg gas outlet.
How big is your plastic keg, 23L?