The ability that defines Brotherhood Vertibird’s power works in all zones, not just the battlefield. As long as Brotherhood Vertibird is on the battlefield (and still an artifact), that ability will count Brotherhood Vertibird itself.
Flying
Brotherhood Vertibird's power is equal to the number of artifacts you control.
Crew 2 (Tap any number of creatures you control with total power 2 or greater: This Vehicle becomes an artifact creature until end of turn.)
