Well, the smartass answer would be “One Bar”. (AKA “one atomsphere’s
worth of pressure”.)
Realistically, that’d be 14.7 PSI at sea level most places. If
you’re in death valley, you might get to 15 PSI. If you’re on a
mountain, not nearly so much.
Most low vacuum system gages are measured in inches of mercury. (if
it starts at zero, goes to 30, and maybe says something about “Hg”,
it’s a mercury gage.) That depends on the pump, but somewhere in the
28.5 to 29.9 range. (If you crack 29.3 inside a minute or two,
you’ve got a good pump)
No matter how good your pump, you can’t get more than that. The
pressure is limited by the weight of the atmosphere pressing down on
the void you created with the pump.