The boy scouts require 6 knots, and teach dozens more, assuming you can already tie a square knot then the other 5 are:
2 half hitches
taut-line hitch
timber hitch
Clove hitch
bowline
Additiinally there are 2 ways to do a clove hitch, depending on the situation. One way is used to tie the end of a rope to a pole, the other is to.tie the middle of a rope to a pole.
The bowline also has variations, such as the aforementioned one handed tying around your chest, which is used for rescues, and tying it on the bight, which makes a fixed double loop in the center of a rope.