Quick answer
[Official] Big Walk does not have a single-player mode. It is designed for 2 to 12 players; you can start alone and walk around briefly, but the cooperative challenges prevent meaningful solo progress.
Choose the world setup
[Official] Each time the host starts a session, they select a 2-player, 3-player, or 4+-player version of the world. Some world elements adapt to that choice. Treat the selection as the minimum expected group size rather than a cap.
If players will arrive late or leave early
[Official] Choose a lower world setting when some players will not stay for the full session. House House does not identify one ideal group size: fewer players create a more focused cooperative experience, while a group closer to 12 is intentionally more chaotic.