Budget snacks for game night.
Snack budgeting for a game night is a per-head calculation across three lines — savoury, sweet and drinks — because those scale independently and drinks are usually the one that surprises. The contribution adjustment matters if guests bring something: each contributor is assumed to cover roughly their own share, so in a group of six with two contributors you are funding about two thirds of the evening. The per-guest figure is the one to use when setting a fair contribution if you would rather split it.
Total spend
Gross = (savoury + sweet + drinks per guest) x guests
Your share
Your cost = gross - gross x contributors / guests
Around 8 to 12 for an evening with proper snacks and a couple of drinks each. Below about 5 you are into single-bowl territory; above 20 you are effectively catering a meal.
Ask guests to bring their own drinks and cover the food yourself. Drinks are the least predictable line and the easiest to devolve, and it is the convention most groups already expect.