This exists in Grecko's API:

https://okcerg.github.io/SortFilterProxyModel/qml-sortfilterproxymodel-sortfilterproxymodel.html#count-prop

It's implemented like this:

https://github.com/oKcerG/SortFilterProxyModel/blob/master/qqmlsortfilterproxymodel.cpp#L31-L34

Without it, there's no way to declaratively set the enabled property of e.g. a button based on a certain amount of items in the model being present.