I typically run my LH games with 3-4 players. I've run games with 5 but that may be pushing it (but is definitely possible). I know Josh has run games with two players quite successfully.
There are a few solo rules out there: Liminal Horror modules tagged 'Solo Rules' - Liminal Horror Library
There are also a few Duet examples out there (one facilitator, one player): Liminal Horror modules tagged 'Duet' - Liminal Horror Library