Does
one want access to more funds? (If yes, keep them open)
Do the current cards have good/great benefits, such as: points, miles, cash back? (If yes, keep them open)
If the credit lines are closed, will that impact one's credit score? (If yes, keep them open)
Our default response is to "keep them open" for the sake of building credit history. Further, it’s a good idea to have available funds for emergency purposes.
If the cards are constantly used, but it is not paid off in full each month, then take the interest
into account. If the amounts are too high, then cancel 1-2 cards to start and verify one's credit score does not drop too low before cancelling the rest.