Since models can't move through terrain in this game, I guess the main thing to consider is that you want your combatants to have space to be able to maneuver and move around.
Other than that consideration, I don't think you can really have too much terrain - or as it is called in this game, too many hazards. I would say that maybe 5 or 6 hazards distributed around evenly makes for a game where you will most likely get a chance to push people into them. It tends to be fun if a couple of them are lethal, to up the danger-excitement factor.
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