On the old NBC Show "Late Night with David Letterman": at least 5 appearances.
Show #187 - 2/21/83 - Sly Stone (performing "Dance to the Music") and Sandra Bernhard make their debut appearances on Late Night-
Show #273 - 8/31/83 - guest Sandra Bernhard gives Dave monogrammed pajamas.
Show #469 - 11/8/84 - Dave leaves the studio after Sandra Bernhard offers a joke about sponges. She sits in his chair until the next segment.
Show #668 - 2/17/86 - Raquel Welch resists valiantly but unsuccessfully to demonstrate "The Standing-A Position" that's in her new fitness book. Later, Sandra Bernhard eagerly complies.
Show #1000 - 7/1/88 - Sandra Bernhard/surprise walk-on Madonna guest appearance.
However, there's no guarantee that this is a complete list. I did a search on TV.com and found the above. I also found a biography (2nd link below) that lists the years she appeared:
Late Night with David Letterman, 1984, 1986, 1987, 1988, 1989, 1990, 1991, 1992
Late Show with David Letterman 1994, 1988 (I think that should be 1998)
So my best guess would be at least 10 appearances.