There are 6 total disciplines, and you must do at least 1 race in each one.
Here is a list of the disciplines, and the earliest stage at which you can unlock/play them.
Rally – Season 1 – Alpinestars Trophy – Event 1
Rally X – Season 1 – Alpinestars Trophy – Event 2
Head2Head – Season 1 – Alpinestars Trophy – Event 4
Land Rush – Season 1 – Brembo Blast – Event 2
Gymkhana – Season 1 – Gymkhana Academy – Event 6
Trail Blazer – Season 2 – Osiris Offroad Rush - Event 2
Any questions? Please leave a comment.