My two were house cats for 4 years as my ex had a 'thing' about letting them out. They now happily come and go but up until then they were very happy being indoor cats and never once tried to escape. Actually took a bit persuading them to become outdoor cats to begin with (though you wouldn't know it now! )

I agree two is best as then they have company and also make sure you get a good cat scratcher, more than one and in different parts of the house if you have the room.

As someone else has said, if they have never been out they don't miss it. My two never even tried to nosy round the door, they would run away if you opened it.