How many psi can you pressurize the gas line (3/4in) from the meter line to the unit?

I wouldn't exceed 16 inches of water column applied to the appliance gas valve.

However, when we pressure test gas lines, per code.

1 An appliance must be connected.
2 All the a-valves, (shut offs) to the appliances are closed.
3 The Gas line must hold 15 psi for 15 minutes, to pass.