All characters, even house owning ones, can have apartments, since apartments are instanced their amount is virtually unlimited. (It is limited, I know, but the limits are rather high and I'm not aware of any apartment building running out of rooms yet across all wards). As for fragile servers, the tenant amount is strictly tied to how much data needs to be saved along the house. However the server already knows which characters belong to which accounts, so no new data needs to be saved. The tenant solution was created to allow sharing house with close friend(s) so the limited amount is small in regards that any larger group of friends can be considered to get FC. Sharing your house with your alts shouldn't stress the servers any more than they already are since you can't be online on more of those than one at any given time anyway.