During the bathing sequence, the tent appears to be normal size from the outside, but it is several times as big in the interior shots.
When the Three Bad Men see Hunter and his men closing in, there are thirteen in the gang. Spade kills one, Mike later shoots two, whom he notches on his gun, and he later blows up seven of them with black powder. That should only leave three, but Bull ultimately faces eight of them in the final shootout.