Your computer will support however many ports there are on the motherboard and the physical space in your case. Performance isn't a problem with SATA drives as each port can utilize it's full bandwidth.
Your power supply has a sticker that tells how many watts there are per rail. You can usually look up the specs for a device to see how many watts it uses. drives are usually in the 5-12 watt range, maybe less for the newer green drives. As long as you aren't using x-fire or sli, you can usually get away with a 400w-550w power supply.
My motherboard supports 8 SATA drives. I have 6 hooked up. This is on a system that has a 550w power supply. As long as I'm not gaming, my system is only drawing around 175w according to my battery backup software. My vid card (HD4850) takes it up higher when I'm gaming.
An easy way to know for sure how much power you are drawing is to use a kill-a-watt