Characters are only allowed a set number of Kidous/Powers/Tube Spells. This number is determined by rank, and will increase as you rank up. This is not to limit people, but rather, so we don't have a Shinigami in training with 100 Different Kidou Spells. This will also make people consider more in which abilities to have, thus creating a more indiividual character, rather then just copying every power you see.
The Limits are,
Rank 1 Character(Example, Humans, Shinigamis(Training), Hollows, etc...) are allowed only 5 different Powers.
Rank Two Characters(Normal Shinigamis, Vizard, Numeros, etc...) are allowed only 10 different Powers.
Rank Three Chracters(Shinigami Officers, Privaron, Junior Quincys, etc...) are allowed only 15 Different Powers.
Rank Four Characters(Vice-Captains, Espadas, Vasto Lordes, etc...) are allowed 20 Different Powers.
Rank Five Characters( All Leaders, Captains, and Masters) are given 25 Different Powers maxium.
Note: Kido Spells, Ceros, Movement arts such as Shunpo, Natural Abilities such as Reiatsu sensing, and more, which are not directly connected to your Equipment, are all considered Powers.
Thus, a Vice Captain would be allowed 20 Powers, a Menos 10, a Privaron 15, and all Leaders 25. If you have questions concerning a rank, ask.