It has been stated that there will be 2,000 people per campaign for PVP, and that once assigned to a campaign, that is yours, and won't necessarily change. The campaign is tied to your guild, friends, etc. So let's say I have a large west coast guild, and through the game's logic, most in the campaign, on both sides, are west coast players... makes sense.
How often will a large number of those people be on at once, playing PVP? I personally don't PVP much, and though I'll be assigned to a campaign, I, and my PVE guildmates (let's say 50) will be occupying 50 of those 2,000 slots (50 of the 666.6 on my alliance,) and NOT be in cyrodil, we'll be PVEing.
And if it's a west coast centralized campaign, could it be that at 3:00am on a Tuesday, very few people will be playing in general, and of those playing, even fewer PVPing.
3:00am on Tuesdays = solo PVP capturing keeps!
I know it won't work out that way, but there's still a lot of questions running through my mind. How active will Cyrodiil be in my campaign, when I associate almost entirely with people who focus on PVE, and PVP just for funsies when there's nothing else really going on? 2,000 people max (as it currently stands) per campaign. Matt Firor has said many times that you're essentially locked to a campaign, so you can really get to know your allies and enemies. If my Alliance has 666.6 (repeating of course) members, how many will be in Cyrodil at any given time? With only 2,000 per campaign, there could very likely be large chunks of time where NOTHING is happening in Cyrodil.