Can there be only a lead singer a guitarist a bassist and a drummer in a rock band?

Defiantly not

Of course a four track is the most common form of rock band and is the easiest to write for but there is no limit to the amount of members that can be part of a band.

The only example I can think of now is the Metal Band "Slipknot." They feature 9 members

(8 members, RIP Paul Grey)