Ideally I'd like 6. I know people with 30+ machine collections. Thing is, not everything works. Just keeping them running, let alone shopped or doing restores is too much for one person.
For me, I don't invite people over to play unless all 4 of mine are running. If I own fewer than 4 then no reason to invite people to just play pinball. With 6 I'd have enough to host people for regular tournaments which is the ultimate goal.