If you're talking guest stars Mary Tyler Moore takes the cake. Had tons of people from Henry Winkler, Mary Fran, John Ritter, Bernie Kopell, Isabell Sanford, Fred Grandy, Louise Lasser, Mary Kay Place among others. And then the following appeared in two or three episodes (not always the same character) Penny Marshall (three shows, 2 different characters), Beth Howland (2 different characters), David Ogden Stiers. Penny Marshall, Pat Finley (two different characters) Eileen Heckart, Sheree North
Then there were all the previously famous people that appeared too like Arlene Golanka and Paul Sands etc
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Now more in line with the OP
On her second show "Mary" she had David Letterman, Michael Keaton and Swoosie Kurtz as regulars but of course the show failed.