cost centre

there are so many cost centres in one company code. i want to know the real scenario. can any one tell me the actual thing. i.e. how many cost centres used in one company code. give examples. and what are the remaining cost centres at the end of the fiscal year. iam in confusion. why because while doing product cost planning we are taking the overheads from the cost centres, at the same time how the system takes the actual values.please help me.