What is the minimum number of players in a campaign?

Obviously the ABSOLUTE minimum is 2, the number you need to play a game. But is there an optimum number for having nice variation and suchlike?

A 2 player campaign would work, but you'll want to have each player play with (at least) 2 warbands.

Yeah two player is very hard work indeed. 3 players is good. Four makes it even better. Not played with more than four, but I would think after 5 it would start to get a bit impersonal for my liking.

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When we use the 'campaign' term about our games, we're 4+. In my opinion, you won't get much from a 3-player campaign, and those we just call open battles, where you can use whatever warband you'd like and challenge whoever you'd like, or we agree on who's to play against who.
I'd say 6 players is fun, but very long and tiring as well, even though everyone enjoys to watch a game. 6 would be a nice minimum, but I find it's too hard to find dates everyone can agree on, and even harder to get everyone to remember and in fact show up.

4-6 I think is ideal, but as others have said, unless everyone involved has a dedicated time to it, it's kinda hard to get everyone's games in. One of the chief problems I ran into with my last campaign was a guy that couldn't meet with us regularly, except to play one other player, and it ended up that he wasn't playing against anyone else in the campaign!

I often played 3- or 4-player 'campaigns'. They were fun, but didn't feel much like a real campaign. Thus I would advise a minimum of 4-6 players for a regular campaign.

Our current card-based campaign includes 9 players, but we rarely meet in one place alltogether - instead the players take their action for a campaign turn and then play possible games within the following 2 weeks on their own.