How to show remaining spots when working with multiple appointment types with different pricing

Scenario:

One boat available on one day. This boat holds up to six people (six slots).  Customers can book one slot, two slots, three slots, four slots, five slots or the whole boat. 1 slot is $10. 2 slots is $15 dollars, 3 slots is $20 - different pricing discounts baked into the pricing when multiple slots are booked. I want to do this: For example, If a group books four slots, I want 2 spots to show as remaining for that day.  How do I do this?  If I make six one person slots it works however I don't have a way to give reduced pricing for booking more than one spot at a time. I can't use Add-ons because Add-on don't reduce the number of spots available (this only occurs with appointment types).