The guilds that fall above the 1000 member mark are less than one-thousandth of one percent of all guilds. – Bashiok

But here’s the thing:

The final piece to building your ideal bank alt is going to be a controversial choice for many of you: what to name your guild. Many fashionable bank alts choose to wear a vanity guild plate. That is another way of saying “a guild without members, which will keep you from getting random guild invites, and will also provide you additional bank space.” – Michael Gray

I’ve got a personal guild, which I’ve also heard labeled as a “Vanity” guild. That is, it’s a guild that isn’t a real “guild” as you’d think about it normally on the game – I’m the only member of my guild, or if there’s someone else in it for a time, they’re only there to help me accomplish something, like inviting my alts into the guild. This guild I have isn’t used for regular guildly things, either.  - WowGrrl