Hard to say exactly since every game is unique. Generally space would be the first extra key normally used for jumping. Though with these web games I've also noticed some issues with that cause of webpage scroll so keep an eye out for that. I think enter is definitely a no-go when used in conjunction with J since you have to shift your hand to use it.
Ideally you want the same sort of feel on the keyboard as you would a controller with minimal hand movement. Imagine if you were holding a ps or xbox controller but you had to stretch out your pinky to push one of the buttons.
For a comparison this is another game from this jam https://zander-games.itch.io/av it had a lot of buttons(was space jkl) but the way they used them it never felt awkward in any situation.