the boss summon locations can vary a lot. I have had 3 of the first boss within short distance of my spawn in before, but never counted them.
I would say 2-5 is probably accurate or close enough to it. maybe its 3-6, but ?? it doesn't make much difference. If you want a better idea of this, I would make about 20 maps online and count them up, see if you can prove you never get 1 or 2 or 6-7 etc.