Seems to me as you radiate out from the center of the map, there would be more room for boss locations to be more equally scattered about, but when they just have 4 queen locations on the outer edge it seems the distance required to get to them is disproportionate to the progression of the others.

You can find multiple summoning points for each boss in a world but bosses are singular. You summon one and then go after it. If you find a vegvisir it marks the closest summoning location for that boss. You can summon the boss from each location but you don't need to there's no special reward for killing the boss at each possible summoning spots. You can kill the same boss multiple times if you want the extra trophies. You can have one on the stones and one on an item stand in your base that way.

all the bosses should spawn on the the world. its just hard to find them all.

"Of each type" - what??? Do you want to know how many of a single boss you can summon simultaneously? Or do you think there's "types" of bosses that randomly spawn in the wild?

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.

You can check for your seed/world online with the Valheim world generator.