As a game jam veteran (being doing jams for almost 7 years), my recommendations on teams are the following:
- If the jam has a lot of participants, try meeting new people, don't team up with people you already know.
- 6 persons or less on a team, preferably 4.
- Going solo? I don't recommend that unless you have a lot of experience under your belt.
- Do something you don't know how to do, You know how to code and don't know how to do the art? Then do the art.
Take in mind that "winning" a game jam has literally no meaning to anyone. I don't even know why game jams became a competition in the first place.