For a single stage compressor,
Volumetric efficiency = (Volume of gas entering the compressor per minute)/(piston displacement per minute)
In a multistage compressor, there are many cylinders, the gas is progressively compressed across many stages. In a multistage compressor, only the displacement of the low pressure cylinder is taken into account. Therefore, for multistage compressors
Volumetric effficiency = (Volume of gas entering the compressor per minute)/(displacement of low pressure piston per minute)