Do all of the Seinfeld episodes have a stand-up part?

The answer is no.

In most of the series, Jerry Seinfeld performed a standup routine at the beginning and end of each episode, the theme of which relates to the events depicted in the plot. In later seasons, these standup clips became less frequent.

In Season 8, the opening (and closing) stand-up comedy segments were discarded, replaced by more conventional opening sketches at the beginning of each episode.

In the last season (Season 9), in the final scene during the credits, Jerry is seen wearing an orange prison suit, doing a stand-up routine of prison-related jokes. The finale was not filmed in front of an audience, for the sake of keeping its plot secret, though a laugh track was later added. It was the first episode since the 7th season to feature opening and closing stand-up acts by Jerry Seinfeld.