How many checklists are there for a commercial pilot for the entire flight?
Typically, the checklists in normal operations are:


External walk around and preflight 
Before start 
Before taxi 
Before take-off 
After take-off 
Cruise
Descent 
Approach 
Landing 
Shutdown  
Secure


This list is pretty much the same for all aircraft.

Normally, each checklist is only used once at the appropriate time but a wise pilot would re-run a checklist if interrrupted.  

For example, you are cleared to start and complete the before start checklist but there is then an unexpected delay before you actually start.  You wait for 30 minutes.  No need to redo the before start checklist because you did it earlier?  No, do it again.  Something might have changed.