There are 20 CTR Tokens in Adventure Mode (Story). Earning all of these Collectibles unlocks the Token Collector trophy / achievement.

You get 16 CTR Tokens from the race tracks in Adventure Mode and 4 come from the Crystal Challenges in Adventure Mode. After winning a race track for the first time you can go back to it and start the CTR Challenge. Your task within these challenges is always the same: find 3 letters "C-T-R" hidden around the race track. They are often on shortcuts and can be hard to see. This video series will show the CTR letter locations for each race including the 4 crystal challenges.

After finding the 3 CTR Letters you also need to win the race to be awarded a CTR Token. It is HIGHLY recommended to do this on easy difficulty. On hard it would be close to impossible on some tracks as going for the letters can make you lose a lot of time. If you started the story on hard, just redo it on easy for the CTR Tokens.
(Side Note: getting CTR Tokens is not needed for the Hard difficulty trophy, you just need to finish the story races on hard to unlock that trophy. Therefore it makes sense to not stress yourself on your hard playthrough with this).

You need these CTR Tokens to unlock the 5 Cup Races in the Gem Stone Valley area. The Cup Races then award you with 5 Gems for the "Gem Collector" trophy / achievement. With this you can reach 100% Adventure completion and challenge the hidden endboss.

Note: The CTR Challenges from Crash Nitro Cart are not needed for the "Token Collector" trophy or achievement.