Collect serving pieces ahead of time. They should not be too large and should be replenished frequently so that the 50th guest in line, for example, is not confronted with a dish that looks as though it had been waded through. Make up two dishes of the same item and when one is almost empty replace it with a full one while the finished one is refilled. Amounts can be tricky. Quantity usually depends on what is served and the number of dishes.
More food is needed for self-serve meals.
Weather can also be an influence. In cold weather people tend to eat more and in hot weather drink more. The size of side dishes does not need to be the same. For example, when preparing three salads for a crowd of 225 people prepare black bean salad for 50, rice salad for 75 and potato salad for 100. The thinking is that more people would be likely to take potato salad.