550EX II

550EX II

if i get 2 and put one on top of cam then one on tripod can i set it to pick up flashes and automatically flash if it sences another flash?

how many can u do it with?

Yes, with some caveats. Read the EOS flash bible. If you want it to flash when it sees any other flash you'll need to buy an optical slave. You can also use the built in master/slave system, which is automatic once you learn how to use it - the manual has instructions.

btw it's 580EX II. They're expensive if all you need is manual flash.

Can't add much more to what Tim's said... getting a few 580s is an expensive way of getting off-camera lighting, though. For some cheaper ideas perhaps check out the Strobist blog.