Almost all Pokemon with multiple forms are counted as one Pokemon. This includes gender differences like Meowstic-M and Meowstic-F, form changes like Deoxys, and even type changes like Arceus. This can be proven by going into the National Pokedex, which clearly classifies different forms as one Pokemon. There are two exceptions to this:
Exception 1 is regional forms. While they're technically the same Pokemon as classified in the Pokedex, they are typically different enough to be considered their own Pokemon. This is chiefly evident in (Galarian) Meowth, Mr. Mime, Corsola, and Yamask, and (Hisuian) Sneasel and Qwilfish, as all are able to evolve into new Pokemon that their original forms cannot.
Exception 2 is the Nidorans. Due to limitations at the time, the Nidoran forms occupy two entries in the Pokedex. While by modern standards they would be considered mere gender differences, they are technically two separate Pokemon.
Tl;dr Deoxys is 1 Pokemon with 4 forms.