I assume you mean off road and not on the street.
For rocks and ruts, 15 to 18 psi minimum, as any lower and you might pop a bead or damage your rim. For sand (beach, dunes), 12 to 15 psi. Sure, you could go lower in each case, but you are taking chances. Try dropping pressure gradually for the terrain you drive until you dial in the minimum you need to get the performance you desire.
If you drive a lot in sand, you might want to get a sand tire that is better suited for very low pressures like 8 to 10 psi.
Of course on the road, you'll need to look at both the vehicle manufacturers recommendation and the tire manufacturers maximum pressure. They will not likely be the same, so you'll need to make a judgement call from there.