How many outlets and switches can go on one 20 AMP Fuse, using 12/2 wire. I am wiring a basement one room at a time, so I to make sure that I don't overload a breaker.

It is not the number of outlets or switches on a circuit that overloads it, its what you plug into the outlet. You need to think of everything that could be used and add up the wattage of ALL items for a worts caes scenario. In basements it's a good idea to have lights and outlets on separate scircuits.