So, with the "recent" Miskatonic Horror expansion and the "Number of Players" guide sheets, there's nothing about the number of expansion boards affecting Monster Surges.  I've always played the game that the number of monsters that spawn is a straight value of either the number of investigators or the number of open gates, whichever is higher, and this straight value is not subject to the rules of expansion boards decreasing the number of players.

A game buddy has finally argued the point so much that I decided to come ask.  When playing with 2+ expansion boards, do you count less investigators for determining how many monsters spawn during a monster surge?

Yes, we do. That is the intent, since the player handicap reflects the fact that the investigators "have more ground to cover." Monster surges certainly count as covering ground.

I play that everything is affected except doom tokens in final combat and required gate trophies to win by closing all gates. This includes "add X tokens per player"-style rumors.