Whatever you feel like supporting, but at a MINIMUM I would include an executable for windows. Personally I usually release .exe and AppImage (linux). I could care less about Mac.

For Game Jams I'd recomend something runnable in browser. Unity, HTML5, javascript etc. Alot of users (and reviewers) aren't comfortable downloading games due to malware concerns so a browser game is more approachable. (FYI please use a virtual machine when installing games made by other people as it can help neuter/contain alot of types of malware)