I noticed when watching Troubled Waters that two of the actors had been other Columbos, Robert Vaugn and I forget his name but he was in Dagger of the Mind.
I know who you mean, he was in The Mummy as well: Bernard Fox.
I don't know which episode has the most, but Last Salute to the Commodore certainly has a few: Richard Vaughn, Fred Draper, Wilfrid Hyde-White and Bruce Kirby. Can't remember if Michael Lally popped up in that one.
Short Fuse has several too: Anne Francis, James Gregory, Ida Lupino and William Windom.
The Most Crucial Game: Robert Culp, James Gregory, Dean Stockwell and Val Avery.
Edited: ah, I've finally managed to get past four. Identity Crisis has Patrick McGoohan, Leslie Nielsen, Val Avery, Bruce Kirby and Vito Scotti.
"Last Salute" has at least three more: John Dehner, plus the curly haired guy who played a coroner in "Friend in Deed", and John Finnegan popped up too.
I noticed when watching Troubled Waters that two of the actors had been other Columbos, Robert Vaugn and I forget his name but he was in Dagger of the Mind.
There's even three of them. In addition to Robert Vaughn and Bernard Fox, we also have Dean Stockwell. He played the murder victim in The Most Crucial Game, and in this episode he plays the band member whom Vaughn's character is trying to frame for the murder.