Beyond just their names, once you start to look into who they are in more detail, you find that they contain several specialty imprints inside them. For instance, one for romance, one for horror, one for detective, etc. Rather like chocolate-chip cookies!
In the sci-fi/fantasy field Locus Magazine's annual yearly review (which comes out each Feb. 1st) lists the number of SF/F titles published as follows for the top 6.
267 - Penguin
224 - Tor
196 - Harper/Collins
141 - Random/Ball/Bantam
121 - Hachette/OrbitUS
116 - S&S Pocket
followed by
78 - Baen
76 - Harlequin
69 - St. Martin's
50 - Disney/Hyperion
49 - DAW
and so on for almost 200 publishers who publish at least a few SF/F titles a year in a mix of hardback, trade paperbacks, and mass-market paperbacks, both new and reissues.