there’s not a rule here. you can have 1 or 10 LAC per BSC.
The only thing you MUST avoid is sharing one LAC over several BSC.
For example, never do this:
BSC A = LAC 1, LAC 2, LAC 3
BSC B = LAC 1, LAC 2, LAC 3
BSC C = LAC 1, LAC 2, LAC 3
That would totally overload your BSC paging processing capacity and perhaps your SS7 capacity on A interface as well.
Then if you put only 1 huge LAC in one BSC, there could be another kind of problem : PCH congestion on the Air interface.
A good bet is to split your BSC in 2 to 4 LACs. That’s just a rule of thumb, it really depends on many many parameters.