Hello I am about to finish the rest of my basement. 2 bedrooms are already completed and were finsihed by my brother-in-law who is now passed away so could not help. I am finishing a family room, a bathroom, and one bedroom. My questions are as follows:
1) When my brother in law finished the 2 rooms, he used the free spaces on the home circuit breaker. The rest of the basement is already wired for lightning, so should I:
a. tap the family room and bedroom lights into the current basement circuit, and if so I know that bathroom also needs to be on its own dedicated circuit, so how should I wire that?
2) Finally will I wire the outlets for those rooms tapping into the circuits that my brother in law installed for the other rooms?
3) Do I just need to get another circuit breaker box and connect the remaining basement rooms to that?
Thanks for your help.

Only you can answer some of these questions. It greatly depends on what is on these circuits now, such as how many lights and receptacles. There is no code limit to number of items, but you must use good judgment when laying out circuits so as not to create a pattern where you are overloading some and under utilizing others.

The bathroom receptacle must be on it's own 20A circuit, but the other items in the bath can be on this circuit if you choose.

To add, the bedroom and family room will need Arc Fault protection.

And, the bedroom will need a smoke detector interconnected with another one outside the bedroom in the common area, and ideally connected to others in the home.

You really should be checking with your local building official for any local codes or changes to the National Electric Code.