If you’re deciding how much punch per person to prepare, start with 2 cups (16 fluid ounces) per guest for a two-hour party, then adjust for other drinks and party length.
Essential Concepts
20 guests: 2½ gallons; 50 guests: 6¼ gallons, before any reserve.
Related posts: https://www.simplyrecipes.com/recipes/eggnog/ and https://www.simplyrecipes.com/recipes/hot_mulled_cider/