Tried creating a playlist with only limited number of episodes but it just keeps playing other episodes of same show.

That behaviour does not sound right. You are saying that if you create a playlist that is literally just Star Trek S1E1 (one single episode), it will play that episode, and then automatically go on to play S1E2, then S1E3, etc.? I cannot find a way to make that happen. In my experience in that situation, it plays S1E1, then the video player exits. It does not start playing S1E2.

That sounds like there must be some setting like "continuous mode" or "autoplay" that is enabled. In other media apps, that sort of setting is sometimes used to continue playback of similar media when the current playlist ends. But as I said, I cannot find a setting like that anywhere in Jellyfin.