Typically, each house has somewhere in the neighborhood of 30-35 students living in it. You can get the basics here.
As far as privacy goes, I guess it depends. You might live in a ‘lawn’ house, like Booth or Dewey, which are centrally located on campus (i.e. the closest to the Dining Hall/mailroom) and not very isolated. Or, you might live in a house at the end of a street, like Welling or Noyes, which generally have rooms facing outwards towards the hills and/or private yards for residents to lounge about in. In my experience, though, the location of your house isn’t really a huge factor in privacy - you have shades on your windows, after all - it’s more about which house your personality fits best, which is a perfect segue into my next point.
You asked about practicing instruments in houses - each house decides upon its own quiet hours (if they decide on any at all) and, in addition to various other group-living rules, policies regarding playing instruments, music volume, etc. A big part of the housing experience on campus are house personalities, which fluctuate from term to term, but offer a very wide range of comfortable living situations for folks of all tastes.
Noise level (and, I suppose, musicality) is certainly a factor in house personality; some houses might cherish the quiet and prefer you practice up in the Jennings music building (which is really not such a bad way to fly); others might welcome a new bassist in the house’s resident garage-punk outfit, and throw an impromptu show on the porch; still other houses might welcome practice in moderation - the happy medium, if you will. There are 18 houses on campus (not including Longmeadow and Shingle) and thus, 18 unique personalities for potential residents to choose from. Having been a student here for a couple years now, I can legitimately say that housing at Bennington offers at least one comfortable and happy living situation for all sorts of different personalities and preferences.