Hopefully the below can give you an idea of sizes and portions
Round Cake | Square Cake | |
6 inch | 20 slices | 25 slices |
8 inch | 35 slices | 45 slices |
10 inch | 50 slices | 60 slices |
These of course are approximations and also depend on who is cutting the cake and also how big each slice that is cut!