No, supply in this case is the number of units available for rent, units being rented are unavailable on the market. Same as real estate market, when they talk about housing supply they don't count houses that are already sold.

In economics, supply is the amount of a resource that firms, producers, labourers, providers of financial assets, or other economic agents are willing and able to provide to the marketplace or to an individual. https://en.wikipedia.org/wiki/Supply_(economics)