The same rules we learned for the M1 and M2 still apply. Display interfaces are generated deep inside the system on a chip. How many there are depends on the exact type of Processor:
• M3 (plain) like the older 13-in and 15-in supports up to ONE fully hardware-accelerated external display.
• M3 PRO processor supports up to Two fully hardware-accelerated external displays.
• M3 MAX processor supports up to Four fully hardware-accelerated external displays.
In every case, turning off the internal display does Not allow you to connect another external display.
Tech Specs now available here:
MacBook Pro - Tech Specs - Apple
Exceptions: if you are NOT using full-motion Video, but only slow-to-change data like stock quotes, program listings, and spreadsheets, there are some hack devices that can give you an additional 'slow to update' "fake" display.
Post back for more information if that applies to you.