Typically each motor added to a bench will have less cfm vs your basic math example.
Example:
8 of the 115923 Ametek vacuum motors rated
well over 100 cfm each will only allow a 600 cfm rating at 25" water on a SF-600 bench.
600 cfm (280 l/s) @ 20" (50 cm) of water
500 cfm (235 l/s) @ 36" (90 cm) of water
Tom V.