Is there a single word for the number of people currently in a room? For example, the room has a capacity of 50, but the current ? is only 3.
If you want a word that matches up with capacity, you can use occupancy for this. Per the OED:
- a. The condition of being an occupant; the fact of occupying; the act of taking, or fact of holding, actual possession, esp. of land (spec. in Law, the taking possession of something not belonging to any one, as constituting a title to it); actual holding of or residence in a place; = OCCUPATION1, 2.
a. The state of being occupied or busy; = OCCUPATION 4.
b. Teleph. The proportion of the time during which a circuit or device is handling calls.
c. The proportion of accommodation occupied or used.
The closest word to what you want is probably occupancy. See the Merriam-Webster definition of it here: http://www.merriam-webster.com/dictionary/occupancy
One of the definitions they give is this:
the number of people who are in a particular building or room at one time