depends mostly on the wand you're using and recovery ability of your machine.
in particular, the angle of jets and air flow your machine creates.
If blowing straight down like an AW-29/PMF 802 style wands, 350 psi is all that's needed.
More than that and you're injecting water/dirt to/thru the backing.
get a piece of thin cheap carpet, and clean it at various psi.
When you find that the backing is getting damp in the slightest, it's time to back off the PSI.
That will tell you what the optimum psi is with "your" wand and machine
..L.T.A.