I believe the minimum should be 5 to start, though I think in that case the number might have dropped to 3 for a few guilds through loss of members.
For Town+ alignment, it's a little confusing, but I'll try and give an example.
If your guild is a good-aligned guild, you wouldn't want guild members to be part of an evil town, because then they could go around doing good deeds for the guild and then start randomly killing and looting, saying "I'm on town business, not guild business."
Likewise, were you in a good town but an evil guild, it would be pretty awkward to have to fight your own town members should a good vs evil conflict ever occur between the two.
So basically, for whatever your guild's alignment is, the members of the guild, if they're a part of a town, need to be in a town with either the same alignment or one that's close enough.