i'm not sure whether i should put more than one level for my game. i'm thinking i won't have enough time to do the second one...

Start with one level and add more when you're done with the rest of the game (if you have time)

Ja start with one level that contains all game mechanics and create prefabs, so you can implement all in other levels to increase difficulty over time. I did it the same way about 2 weeks ago in another game jam. If you want to you can check out if you want to, but  it don´t fits the of brackeys theme. 

https://iceman2708.itch.io/walters-nightmares

Dont give up, stay focused and  don´t forget a break from time to time.  :)