All together, there are 11 obtainable outfits in BDSP (or 22, since there is one particular outfit for each gender).
These outfits are:
the Everyday Styles (the clothes that you start the game with)
the Platinum Styles (obtained as a mystery gift by pre-ordering Brilliant Diamond or Shining Pearl or by purchasing the game before February 21st, 2022)
the Pikachu Hoodie Styles (purchasable at the Metronome Style Shop in Veilstone City)
the Overalls Styles (purchasable at the Metronome Style Shop)
the Spring Styles (purchasable at the Metronome Style Shop)
the Cyber Styles (purchasable at the Metronome Style Shop)
the Winter Styles (purchasable at the Metronome Style Shop)
the Casual Styles (purchasable at the Metronome Style Shop)
the Eevee Jacket Styles (purchasable at the Metronome Style Shop)
the Gengar Jacket Styles (purchasable at the Metronome Style Shop)
the Leather Jacket Styles (purchasable at the Metronome Style Shop)
Source
Hope this answers your question!