That's difficult to answer without knowing the application - which includes how far the servos are moving, how many are moving at the same time, and what weight the servos are holding. Don't quote me on a number but I've seen 4 servos draw as much as 4-6 amp spike on a small robot. i think most users are aiming for more than 10 amp power supply - and of course this doesn't mean linear voltage reduction - digital switching is a must.

If this is for desktop (classroom/lab) use, I would recommend 20 amp adjustable power supply. There's plenty on eBay, which are the ones we use in the lab.

Yes it's for classroom use Thank you DJ

If that is the case, a bunch of these would be very handy: http://www.ebay.com/itm/10A-30V-DC-Power-Supply-Adjustable-Dual-Digital-Variable-Precision-Lab-Grade-/251705177148