There are 12 Red Bricks total to get, and you get 1 per Pixar Family Build.
The Pixar Family Builds are unlocked when you complete a Crime Wave for a district. After the small cutscene showing the ending of the crime wave and showing the Pixar Family Build appearing, you will regain control directly in front of said build.
Build the Pixar Family Build to receive a character bag with a character from the Pixar Movie reference in the build (Dory, Lightning McQueen, Woody, Wall-E, etc). Use your new character to interact with the build set and you will unlock the red brick contained within. (You don't have to use the newly unlocked character but they will always have the ability needed to interact).
There is no purchase price once you discover a red brick, you simply have to activate them through the Triangle Menu.
Push triangle, then press L1 (or R1 3 times) to get the the Red Brick menu. Highlight the brick you want to use and hit X.
Once you have completed all crime waves a final Pixar Family Build appears in the Financial District for your final Red Brick.