You can't do that directly. If you need this count, you could create a field on the Person class (i.e. numberofbelongings) which you update in an afterSave cloud code hook on Belonging. Whenever a Belonging is saved, you also increment the numberofbelongings field on the Person object, and save that as well.
No extra queries, no extra network traffic from your device, and your always have the count available for any Person object.