Including the GM, you can play with any number of 2+ players. I'd probably recommend no more than 4 or 5, but that's my typical recommendation for any TTRPG.
Average play time for this game can be pretty short. It depends on the number of players and the length of run you choose, but I've run one-on-one sessions that were only 45 minutes to an hour. The GM has some tools they can use to increase the length if they wish.
There's not really a solo mode in that I do not provide any story for you to play through. That said, I did some playtesting via solo play and did have some fun with it. I think you could certainly create a run with solo play in mind.