Needs a Better Title
. There are some shows which, by virtue of being long running and having an ever changing cast of incidental characters, need to hire a LOT of actors. So many, in fact, that after a while it seems like practically working actor in the country has appeared on the show at one time or another, sometimes several times.
In the UK, examples include The Bill
Not sure what US examples there'd be.
Note: this trope is NOT about shows who have managed to rope in big stars in high-profile guest roles (e.g. The Simpsons). It's for shows which have dozens of transient bit parts, and which turn up early in the CV of practically every actor in a given country.