brunch for sixty people - how to estimate amounts?
There are two parts to your calculation: how much egg or tuna goes into a "helping" at your buffet brunch, and how many helpings you are likely to need.

For the first part, you probably know how many egg salad sandwiches you can get from a certain number of eggs. Let's say one egg makes one sandwich. Are people likely to put that same amount on a bagel? Or will it be more? Less? Let's say a can of tuna makes two sandwiches. Same questions. 

Next, how many bagels-with-something will a person eat? Maybe two, right? Especially since there is kugel. So 120 bagels would be a LOT of bagels. You could then take your guesses from the first part and estimate what you need for 60 bagels of egg and 60 bagels of tuna. Sure, one may be more popular than the other, but some people may eat only the lox, right?

If you've never done this before, I wouldn't make up 60 bagels of egg salad mix in advance. You can make maybe 20 worth, and as it disappears, make more. That is assuming a relatively informal get-together where you are the host and friends and family are happily gathering, and will watch you stir up some more egg salad (using eggs you hardboiled in advance, and veggies you chopped in advance) as needed. Things would be different if you were the paid caterer. Not doing it in advance means that if you don't use all the eggs or veggies, you'll have a chance to use them in something else instead of egg salad. Same for the tuna - open the cans as you need them after some initial inventory of tuna salad is made.