Section 906 of the IBC (2012 edition)--identical requirements are in the IFC.
You need to classify the classification and hazard of fires anticipated.
With a commercial repair garage, there is a potential for liquid spills that will be 1/4-inch or less in depth. Therefore, an extinguisher with capacity for handling Class B fires is required. Class A fires are also a potential.
Repair garages are considered ordinary hazards by NFPA.
For Class A fires, IBC Table 906.3(1) requires a minimum 2A extinguisher with coverage of 1,500 sf per Unit of 'A', or 3,000 sf. Travel distance to a fire extinguisher is 75 feet. However, this travel distance will be reduced for Class B fires. Per Table 906.3(2) a minimum 10B extinguisher at 30 feet of travel distance or a 20B extinguisher at 50 feet of travel distance is required. There is no area limitation based on Class B fire hazards.