The "commercial" property sector is broken into various components, which would include retail, office, industrial, hospitality, etc... The measurement is rarely in terns of "number of properties" and is typiclaly in terms of square feet, or rooms, or units, depending on the specific asset type.
Many of the large, national brokerage firms will publish annual surveys which may summarize the total space absorption in a given year, for a given property type in a given geographic market. One such report that I have in front of me details the total transaction volume (sales volume) by property type. It looks like nearly $20B in office product changed hands nationally in 2011, for example. The same report details 55 national CBD's and shows a total CBD office inventory in these 55 metros of about 1.6 billion SF, and a forecasted net annual absorption of about 10MM SF between 2012 and 2014.
So, to answer your question, you would likely need to refine your question to get a more accurate answer.
Feb 7, 2012