Positive Pressure Ventilation

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What is the leak flowrate at 500 pascals?

This is a function of the building design.

if we maintain 20000 cfm supply and 15000 cfm exhaust in 12350 cu ft room,how much positive pressure would be inside the room, is that abt 500 pascals???

With those figures, the room would increase in pressure until something blows out, like a window, a door or even a roof or wall. The doubling time is 12350/(20000-15000) ≈ 2.5min and 0.2 bar is enough to knock a wall over, so expect something to go BANG in about 30 sec.

No, the incoming and the exhaust rates are the same. What are they at 500 pascals? Once that figure is known, the rest falls into place, because that is the figure needed to size the fan. "Air changes per hour" is a useful number. What is it, please?

Once again, it depends on how tightly sealed the room is, how often and how long doors might be opened, etc. Even if you can calculate these things, actual installed conditions may vary considerably. One approach would be to size each fan generously, and then use a VFD to control one or the other so as to maintain the desired 500 Pa pressure difference.

Is the ONLY criteria that the space be pressurized to the level indicated? With no concern for temperature or air circulation rate?

IF you only need to pressurize the space, the easiest way is to install a supply fan sized for, say, two air changes an hour (12350 x 2 / 60) = 412, say 420 SCFM at 600 Pa (about 2.5 inches of water) and install an adjustable barometric damper at the exhaust from the room (venting to atmosphere) sized for about 500 FPM airflow velocity (about 120 sq inches). Adjust the damper for the required pressure.

IF you need to maintain a temperature in the room, have that system operate with recirculated air only, with capacity to support the supply air at whatever conditions it is- temperature and humidity to the room conditions.

If the room will see any amount of door openings during an hour, you might want to boost the fan size to twice what was suggested earlier, with a correspondingly larger exhaust.