I have to say, i don't really see the need for such precise definitions.
Does it really matter if there are 120 or 200 vamps in a zone ? Once that kind of opposition appears the characters won't count them in any case. It's just lots and lots of vamps.
As for how many can attack at once... if i want a group of characters to face 200 Vamps i obviously want them dead, or force into submission or whatever, in any case Conflict rules don't really apply now do they ?
I think what matters most here is the question: How many attacks do you wan't to roll and resolve per round ?
Because as a GM i sure as hell wouldn't want to roll 200 attacks... or as a player wait for my GM to resolve 200 vamps attacking me.
I say as many can fit and fight in a zone as is dramatically appropriate and common sense allows for. No rules needed.
And like i said, at some point true numbers really become unimportant.
As for how much space a single person needs to fight.
I'd once again keep it simple. Unarmed, Handguns, Knives, Short swords and similarly sized weapons have little to no trouble in close quarters.
Long-Arms, Swords, two Handed swords and the like might have problems when used in close quarters, provided it's interesting from a story perspective.
"It's really cramped in here so you cant use your M16, you'll have to use your sidearm instead. Here's a fate point"
"You're running around in a tight group, you're afraid to injure your friends with your Claymore. Use your Dagger instead. Here's a fate point"