How many keys should be on the keyboard if I would like to get one for rock music?

Keyboards come in different sizes because people have different needs. There is no rule that classical players must have 88 keys, and there are many rock musicians who play keyboards with a full 88 keys. It just depends on what you're comfortable with, and what your specific needs are. Standard pianos have 88 keys, so if you are playing a lot of piano parts, in whatever context, 88 keys are usually preferred.