I think the current terminal capacity is around 3 million but planning consent was granted for an extension which would probably take it up to 4 million plus. I don't recall the original terminal planning consent stating a maximum permissible number but maybe there was one.
A further plan was published in 2019 which would have involved building a new arrivals terminal and the plan for that showed each terminal having a capacity of 7 million (maybe there was deliberate ambiguity there) with 21 contact stands in total. The total movement cap is 53,300 p.a..