If you are planning a small facility (100-200 potential users), you will need at least 2 different types of major cardio machines: treadmill, bike, elliptical or stair climber. You will also need to cover the major muscle groups with strength training exercises. A multi-station machine will cover your minimum requirements.
A medium facility (200-400 potential users), will need at least 4 cardio machines, two from each category of the following: treadmill, bike, elliptical or stair climber. For strength, at least 4-6 combo machines or a circuit of 6-8 single station machines.
A larger facility (400 + potential users) should have a minimum of one cardio machine per 100 potential users. For strength you will need a complete circuit of 8-12 single station machines plus free weights.