28 Japanese films, 1 American, 1 Americanized (I counted both versions of the original film.) Thanks to @blackholemovies.
Godzilla 
Godzilla Raids Again 
Godzilla, King of the Monsters! 
King Kong vs. Godzilla 
Mothra vs. Godzilla 
Scott Weinberg 30 films
28 Japanese films, 1 American, 1 Americanized (I counted both versions of the original film.) Thanks to @blackholemovies.