Custard - How much milk can one egg set?
It really depends on how thick you want it.  Some sites recommend one egg or 2 yolks per cup of milk.  Ruhlman mentions 2 eggs per cup as 'standard', with 1 egg able to thicken 3/4 of a cup of liquid (but more fat helps).

I can't comment on thickness of creme anglaise -- I had to gave up dairy years ago, and that's not something that I've ever made.