20 would be a good start...
I'm doing kid classes this summer. The kids are 18, not 14 like last time I had to do this. And they're supposed to be "normal" kids.
I get to do three sessions a week with these kids. We're going to build some computers, install Windows and drivers and software, and then make sure everyone knows how to do basic computer things like copy files and burn CDs.
There will be at least sixty and possibly as many as one hundred of these kids.
Anyway, I was given a budget of $600 per kid to get a computer + everything else. This is what I came up with:
AMD Sempron 3500
Biostar nv6100 motherboard
1GB DDR2
80GB Hitachi hard disk
Foxconn mini-tower
SATA DVD Burner
Logitech Keyboard and Optical mouse
17" Acer LCD
Crappy HP Printer
Windows Vista Premium
Total per PC works out to $591. There's actually no place I could make an improvement that would not blow my budget. I'd love to go to an x2/3600, but I can't (nor a Pentium D dual-core 2.66GHz machine - the motherboards cost too much even if the CPU price is right). If I move up to a 120GB drive, my price blows up. I'd love a better case but experience shows that Foxconn is the best low-end option available. And I can't dump the printer, even though that would make for a vastly better PC.
I will note that something similar from Dell (e531, Sempron 3400, 1GB RAM, 160GB drive, 17" LCD, no printer) is $759, shipped; I'm still looking really good, pricewise.