Hi!
Oh, there is a little misunderstanding here. I have not suggested to create a property "number of sitelinks in this document". What I propose instead is to create a property "number of sitelinks for the document associated with this entity". The domain of this suggested property is
I think this is covered by https://phabricator.wikimedia.org/T129046 - which seeks to add page props (which already have sitelinks count I think but we can define any that we want) to RDF. I kind of neglected it due to the lack of demand, but it should not be that hard to do.