Based upon the size of the suppression area you have (cubic feet) or floor space (square feet), than you need to appropriate the amount of fans needed to provide the evacuation of the space.

Don’t forget that you will need to provide fresh-air in, as you exhaust polluted-air out to exchange the air until it is ‘clean enough’ to allow habitable human re-entry (without NIOSH filtered breathing air masks)

The following is curtousy of Industrial Fans Direct.com, I use them frequently! Thank you!
CUBIC FEET CALCULATOR
http://www.industrialfansdirect.com/CFM.html

Use Intake Type and Exhaust Type Ventilation Fans Properly:
There are two types of ventilation using electric powered fans: intake or supply type fans and exhaust ventilation type fans. The combined use of intake and exhaust can increase greatly the efficiency of ventilation

Here are some tips:
1. use intake or supply fans to provide a strong air flow which can reach a distant place. Use intake or supply fans if there is no risk of scattering airborne contaminants.
2. use exhaust type fans to collect vented substances. Use it if there is a risk of airborne hazardous substances being scattered. The distance between the source of the contamination and
the inlet of the Exhaust type fan(s) should be in close proximity. Suction power decreases exponentially as distance increases and is therefore weaker as one moves further from the opening;
3. a combination of intake / supply and exhaust type fans is the most efficient form of ventilation, although it can be costly to set up. This method overcomes the weaknesses of separate intake and exhaust type ventilation. intake / supply and exhaust ventilation can provide strong air flow to distant places without spreading contaminants. The capacity of the exhaust fan must be bigger than that of the pushing fan or contaminants will be spread throughout the workplace. Usually the capacity of an exhaust fan must be approximately 5 times greater CFM than that of a intake fan;
4. place sources of heat or sources of airborne hazardous substances close to exhaust ventilation fan(s) to ensure sufficient airflow for their removal.

Facility Guru
Austin, TX