Etsy is a good source for pillow covers

4-5 pillows where ? On that sofa max 2 . IMO if you have to move the pillows to sit down thre are too many . IMo after you choose wall art and all other elements then choose pillows

Decide on what accents colors you want

Max 3 cushions and should be on the small side, mix patterns and include a lumbar shape.

Could add a throw and just two cushions on one side for a nice effect,

Head to Target and/or Marshalls and bring home to test them together. Return and keep trying until you like the combination.