With only 5 fingers, it's possible to count up to 31. 1 is the thumb (left handed) or pinky (right handed). From that, right to left, your fingers would represent the numbers 16, 8, 4, 2, and 1. This is a counting system known as binary, used in almost every electronic item.
That's pretty much about it. Once you start imagining your fingers as these numbers, it's easy to count by showing certain fingers and adding the numbers. For example, the "hang loose" gesture represents 17. But, please avoid showing the 4...
If you added your other hand, you could count up to 1023. But be careful with 128 and 132!
If you added your toes, you could count up to 1048575 (2^20-1).
If you brought a friend over, both together can count up to 1099511627775 (2^40-1).