You can use as many as you want, but keep in mind the F-18’s third MFD isn’t really an MFD in BMS. It’s still based on BMS’s F-16 avionics, so the buttons are used for various F-16 things like autopilot and enabling the chaff and flare systems. Same keyboard commands and everything (besides the few specific to the F-18 in BMS, like autothrottle and takeoff trim), just a different place in the cockpit.
BMS does have keys for a third and fourth MFD, but to my knowledge there still isn’t an aircraft in BMS that uses them.