Players can gain more than 2500 ranking points - cups by winning duels.
Winning duels will let you gain more cups but every 2 weeks all players who have more than 2500 points will have their points set back to 2500 and the surplus of extra points above that number will be added to their Legendary Trophies points.
Your current Legendary trophies points can be viewed on your Profile.
There are currently no special rewards for gaining Legendary trophies.
You can find more info on Duel Ranking points here