I don't think Rising Damp would have worked as well with more characters. Rigsby's closed mindedness was brilliantly fed by him having just a handful of lodgers to base his view of the world upon. Not quite enough to abuse his vulnerability, but just enough to allow him to feel superior.
Having a wider source would have, surely, led to him gaining a little more perspective on the reality that existed outside his insular little realm...and given us a Rigsby who might have been tempted to look further than his own nose.
He was, without realizing it, being purposely left to stew in his own juices by the characters around him. He enjoyed the idea that, as their landlord, they accepted he was in control...even if, sometimes, he knew he wasn't. They enjoyed winding him up and watching him go. More characters would have diluted the 'boiling pot' situation that made the series so successful.