We do not have an accurate count
From the movie, there is at least one, the Fulminatrix, but in the first issue of the Journey to the Rise of Skywalker comics, we see another one, which is used to destroy a planet's population who supposedly helped the Resistance. In short, we know of at least two dreadnoughts of the Mandator IV-class belonging to the First Order. Another one, and the only ship of the Mega-class was the Supremacy, Snoke's flagship.
Moving on into the not-so-far-away future, the Xyston-class star destroyer had an axial superlaser that was similar to the orbital autocannons on the dreadnought. In fact, according to Wookieepedia, the First Order dreadnought was a testbed for technology that would later be used in the Xyston-class ships.
Sources
starwars.fandom.com/wiki/Mandator_IV-class_Siege_Dreadnought
starwars.fandom.com/wiki/Xyston-class_Star_Destroyer
starwars.fanodm.com/wiki/Supremacy
starwars.fandom.com/wiki/Mega-class_Star_Dreadnought