I think the normal number for starting and keeping a guild active should be kept. I'm thinking of starting one, and as I don't know of how popular/unpopular it could be, don't want to be left out in the cold.

However, a guild house is more for the successful, rich guilds; status and what not. I'd be all for a larger minimum number of guild folk needed to maintain a guild house. I mean, why would a guild of 5 people need a house to do everything? All they'd need is a pow-wow in someone's house. Not to mention needing to keep it tidy and in relative order. Now, with 10 or more, they might need more structure, and thus, a larger more official place.
Edit:
It's the same idea for watching guild activity. Perusing the guild area yielded people saying which guilds had gone by the wayside, or things concerning having all those alts in a guild. With saying the 5/10 guildies needed, it's saying active players. Now, if someone wants to stock 20 alts and play every character regularly in order to maintain the activeness of the guild, that's fine (but somewhat impossible if someone actually wants to get somewhere character-wise), but I doubt that'll happen.
Remember, active members, not ones that hop on for 3 minutes a day to fake being active.
Besides, could make for interesting RP if people notice it.
'Wait, 20 people are in that guild... why does only one come out at a time?'
'They're witches! Burn them!'