There aren't that many denominations. The way they get that number is counting every single "group" completely separately even though plenty of them are in communion with each other, only separated by a national boundary, etc.
>>512282285
No, they're counting every single "group" completely separately, so for example every single Orthodox church is counted as a separate denomination, even the ones that are in communion with each other, which is the same thing they're doing with Protestants. That's how they get the humongous number.