There are 2 different types of tickets in Scrabble GO. Tickets that can be found in chests (received in Arena tournaments and other game modes), you can only collect up to 5 of those tickets.
The second type of tickets are the ones you get on a daily basis when you are a Scrabble Club member and in the chests you purchase from the shop. You can go above the cap ticket of 5 with those tickets.