I also do not like crowded arenas. Not so much for myself, but for all the IDIOTS who seem to have no sense of arena etiquette.
When I first got my horse Red, he did NOT like ANY horse passing him at any speed at any time. Now I've gotten him to a point where he will tolerate it but he still doesn't like it. So I usually go do my own thing and avoid the arena if it is packed; because I don't like it either.
Ultimately, it is up to you. If you don't want to bother with over-crowded arenas, you don't have to. Don't!
But if you want to work on your horse and get him accepting of it, then go ahead and work on it. Are there other times when the arena has a few riders in it, but not over-crowded? I'd start there. Or if you can have a friend work with you, who purposefully rides around you (in a way you can both control), to work on your horse's responses to another horse.
Just take it small steps at a time.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.