The counsellors are not normally a "tipped" position. However, they certainly appreciate small gifts such as phone cards. The bottom line becomes that any amount you feel is appropriate is fine. We generally bring several $10 cards. Most cards can be used in the US or internationally--you don't need a different card. It just changes the number of minutes available for use.
There are 10-12 counsellors in each kid location. Therefore, I always ask DD if anyone made her time special, and give the cards to the people she names along with a thank-you note. If that counsellor isn't around on the last night, you can leave it at the desk.