Most of the factory schemas in Adobe Campaign use the same sequence of ids for the primary key (XtkNewId). Custom schemas developed during the project phase will also use the same sequence of ids unless a specific sequence has been defined for them.
Therefore the more schemas there are using the out-of-the-box sequence, the higher the consumption of ids will be.
A high ID consumption very probably indicates a problem and should be investigated, the data import procedures being the likely culprits.
Note that in some cases, negative IDs can be supported.