If you want to give yourself the highest chance of capturing as many beautiful moments as possible, I think 2 photographers is always a safe bet. Granted, if you are working with a limited budget and can only work with 1 photographer, that is plenty and you will get some awesome shots undoubtedly. BUT ultimately, someone can’t be in two places at once and when one photographer is taking pictures of people on the dance floor, the other can be capturing candid moments around the party.
Next time you are at a wedding look around and notice how many photographers there are. The last wedding I attended I counted 4 different photographers and 2 videographers! The reality is, there are so many beautiful moments unfolding around a wedding, that it is almost impossible for 1 person to capture everything. If you are hosting a large corporate event, or get together, the same ideas would apply. During a wedding, corporate event, or even some larger parties, schedule’s are so fast paced that a single photographer would have a hard time keeping up and being able to be in the right place every time. I would say a wedding is probably the only event you would ever need 4 photographers. If you are hosting a birthday party, graduation, or a simple get together, one photographer would be plenty.
It also depends how many pictures you want to have from your party, wedding, corporate event, etc. It varies how many usable pictures you get from a photographer, but you can typically count on anywhere from 20-50 photos that are framable, top notch pictures. If you want to get this number closer to 100 usable pictures, it is probably a good idea to hire another photographer. If you want to stay safe, and you have the budget for it, I think two photographers are a great starting point for any event, large or small.