The standard 5E party encounter 'challenge rating' assumes 4 characters - but there are rules to scale encounters. I'm happy with 4 (played through with two different party combos, both worked fine) , but if they went with 5 that would be cool too (we play with 5 chars in my table top group, because there are 5 of us..if there were six, we'd play with 6). But they need to make a decision and the balance the game around the ''recommended party size".