It really depends on the aircraft. If you fill every seat the amount of luggage that can be carried is pretty limited for most light twins. Most if not all charter outfits will fuel the aircraft for a round trip due to the high cost of fuel ar EIS. This further restricts load.
If your charter outfit asks your weight but does not actually weigh the passengers they can gain a substantial amount of payload. The airplane probably won't fly on one engine using that method but it's common.
If you expect to bring a lot of luggage it's best to charter a aircraft with more seating than you need. That will give you the baggage weight needed.
G