Where can a summon be placed in Frosthaven?
In a different empty hex next to the summoner, one summon per hex. If no adjacent empty hex is free, the summon is simply not placed.
Every summon has to go into an empty hex adjacent to the summoner, and two summons cannot share a hex. If the summoner is surrounded, the summon does not appear at all — the ability still counts as used.
The rulebook only asks for an empty hex. Difficult terrain is a property of a map hex rather than an overlay tile, so it is not excluded; an obstacle or another figure would be.
Summons do not act on the round they arrive. Their card goes to the right of the previous summon's card, and they act in that order from the next round on.
“These figures, known as “summons,” must be placed in different empty hexes adjacent to the summoner. If there is no adjacent hex available, the figure is not summoned.”
Rules change with errata and with digital patches. If your copy of the book disagrees, the book wins — the official Frosthaven FAQ is the next place to look.