How many different exercises should I use in a 5x5 program?

It depends on what 5x5 program you are using. If it is the one outlined in the Poliquin principles, then it is typically the same exercise and the idea is to advance in the "cluster" by increasing weight ... for example, first week of bench press might be 5 x 5 at 200 pounds, second week 5 x 4 at 210 pounds and third week 5 x 3 at 220 pounds, then the fourth week goes back to the 5x5 grid but this time at 210 pounds.