Actually, there are at least 20 if you count differing skill levels of the same engine. There are 20 different skill levels of Komodo available as Computer1 through Computer20.
I believe there is another level of Komodo available that is full strength.
Plus Stockfish 8 is used for analyzing.
Plus there is the Play->Computer option. That also has 20 levels. I don’t know if they are the same as Komodo.
how many chess engines are there on chess.com