The world’s busiest airport, by passenger movements, is Atlanta. There are only two terminals, North and South. The world’s busiest airport by aircraft movements is Chicago O’Hare; there appear to be four terminals called, rather confusingly, One, Two, Three and Five. Presumably there was once a Four, but it is no longer used.
The world’s largest airport by sheer size is at Riyadh. It covers 81 square miles, more than half as much again as it’s nearest rival (Denver). There are four terminals, but only three are used.
Conclusion: number of terminals does not correlate closely with size or volume of business. Probably it has more to do with the historical development of the airport and the financing models used for development. JFK has so many terminals because each of the the (then) major airlines built, paid for and operated its own terminal. The terminals are now called One, Two, Three, etc, but they used to be known as the Eastern Airlines Terminal, the Pan Am Terminal, the TWA Terminal, and so forth.