Limiting the number of importances in a list for e.g. books

It would be great if Libraries were able to have the ability to limit the number of importances that could be selected for certain types of resources (e.g. books). We use a purchasing formula and stipulate that no more than 4 Core books can be added to a reading list. We do this so that we have good control of the reading list budget but also so that Academics prioritise what resources are most important on a list for students. However, frequently academics exceed the limit and list reviews cannot be completed whilst awaiting lengthy discussions about what should be set as Core. It would help massively if we could had the ability to set what the limit should be.