*Theoretically, any number of players + 1 GM. In practice, it works best with 1 GM + 3-6 players.
*Average play time is entirely up to the group and what you choose to do during the playtime. In playtesting, an average combat encounter has taken around 15-20 minutes. I've run sessions between 1 hour and 4 hours that all felt like they were useful lengths, but it would probably be hard to get a lot done in under an hour.
*There is no solo mode.
As a side note just to make sure you're aware, this game is currently a playtest version and is not a completed product. I do plan to include this kind of information and some examples of play in a final version, but we're a long way away from that right now. I am working on the bestiary currently and hope to have an update relatively soon, but my day job is not giving me a lot of time to work on it so 'relatively soon' is likely to be measured in months.