My advice is to first decide to do a jam, have an idea of what you want to do (maybe something to build up a specific skill or test out an idea) then go browse and find the one that gives you what you want. Some jams I have a lot of love for are LowRezJam, Nokia Jam, FFS (Finally Finish Something) and *friend of the jam* Virtual Nomad has Olygames starting soon (https://itch.io/jam/olygames-2026).
Ranked jams are not really ruthless because most jammers are super friendly and helpful but it can suck a bit to pour your heart and soul into something for it to end up with a low score. Of course, if you do really well, it feels awesome! Either way, it's the feedback that your game gets from other devs that will help you to see its strengths and weaknesses or see where you need to improve.
For the 20s jam, you can update your game as much as you like, any time you like, for any reason. If you decide you want to expand your game and remove the 20s time limit, we ask that you leave a 20 second version available so people who find it through the jam will be able to experience it that way. By the way, we DO encourage you to expand your game and if you do, you can keep showing us your progress on the 20s discord, even if it's 20 hours.
For other jams, it depends. Ranked jams, especially ones that give prizes, usually do not allow you to update your game until the voting period is over (that's often 1-2 weeks) and then you can do what you like.