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!