So you don't give away the secret unintentionally. It's mostly the Secret-Keeper who will have to be careful. I'd suggest someone outside your house just so they won't have to worry about Apparating- er, Teleporting into a very specific place every time they enter or exit the house.

[She listens and hesitates.] Yes, it's possible to have as many Secret-Keepers as you want, but... with each Secret-Keeper, the risk for the secret getting out is multiplied. It's up to you, really, but they'll need to be there when I perform the charm.