You would need to further filter for the largest railway stations, perhaps by number of platforms ≥ 6 or 8, otherwise you will include many urban and suburban stations that serve essentially the same purpose as metro stations.
I'd say mostly Montparnasse, Gare de Lyon, Gare de l'Est, Gare du Nord, and Austerlitz.
RER, Transilien, Metro and Tramway stations would not count as "train stations" in the eyes of a Parisian/French person, and places outside of the Périphérique (the ring road that follows the old defensive "Thier walls" of the city) would not count as "Paris" for this kind of conversation.
I checked the link in the sibling comment[0] and just found out that "Gare Paris Bercy Bourgogne - Pays d'Auvergne" is a thing that exists. Never heard of it before.
So all in all, from a Frenchperson's perspective there are 7 "gares" (train stations) in Paris.