Type /clock it tells you how long until the Leves reset then you can get a new set. You can get 8 regional and 8 local so 16 in total every reset. They reset every 36 hours. If you have done all the Leves of a certain rank and still have not used all 8 for that reset try a different city to collect some more.

For example if you want to do R20 Battle Leves then you can get 3 from Limsa, 3 from Ul'dah then you have 2 left which you can pick up in Gridania.