Is this a serious question? If so it’s worse than the ‘what tyres’ threads.
Anyway, assuming it’s not. Depends. Depends on how heavy you are, the geometry of the frame, the tyres you’re using, the rubber compound in said tyres, whether you’re using a 26 or a 29er or something else (shudder) the moisture content of the ground and atmospheric pressure. And that’s just off the top of my head.
Start at 35psi and if that’s too soft and the bike feels ‘squirmy’ put some more in, if you’re finding the bike loosing traction on climbs and corners etc, take some out.