[DMK]

Well... we have four.... now mind you together they only weigh as much as one larger dog, but they are a neverending source to fun. We presently have my mildly retarded Jack Russell, and three Rat Terriers.
Over the years we pretty much have always had at least two at one time.
Thet keep each other company and play off of each others induvidual personality.
The only concern is the initial introduction of the new dog. We've found it best to have the time to spend with them, even a weekend of nothing else on, that way you can monitor things and they will set their own pecking order.
Good luck and stock up on dog cookies.