During Initial Registration:
You only need to submit 2 names at this stage. It's only during your final submission before 30 September, when you need your final team members in it.
While there have been plenty of successful solo founders, it is so much harder to build a viable company by yourself. Moreover, convincing a co-founder to join you boosts your credibility, in terms of both the worthiness of your idea and your persuasion skills.
So, we require a min of 2 members, max 5.
Am I stuck with my team forever?
Not at all. Founding teams change all the time, especially within the first year. You can still change your team members all the way until you submit your pitch video. From then, your members should be the same all the way up to the Semi-Finals and Finals.