One quick way is to go to Hosting/Calanders/ click on the property you want the Proerty Number for then look up at the URL line once the Calendar opens. The first long number after manage-listing is your property number. Example:
https://www.airbnb.com/manage-listing/987654321/calendar
Also going to Setting/Profile then scrolling down to your Hosted Properties and clicking on the property you want to know the Property Number of will also give you the answer in the URL Line. After clicking on said Hosting it will display:
https://www.airbnb.com/rooms/987654321
The number after rooms is your Property Number