Re: Film containing the most actors who appeared in a Columbo episode?
Wow, that's quite impressive.
A while back. I asked a related question, but limited it to TV shows.
The most I came up with was 5, and I found several examples. I was a bit surprised that one of them was the Bob Newhart show, but their were four "regulars" on that show that appeared in Columbo, so maybe not so surprising.
Re: Film containing the most actors who appeared in a Columbo episode?
If anthology shows count, there's a huge number on THE ALFRED HITCHCOCK HOUR and THE TWILIGHT ZONE (including Peter himself), and a decent number on THE OUTER LIMITS. I'm not so sure about others.