RPM for airflow equation

In his example to calculate airflow he used an rpm of 6000 from a 350. I was wondering, does that mean when we use the equation do we plug in a max rpm our engine can produce?

If you would please link to, or copy, the equation you have in mind? I would hazard a guess that it is illustrative and used as an example, especially as the efficiency of an engine falls off above the torque peak.