Online today is Mike O'Brien, Rob Pardo and Bill Roper. Remember that we are only answering Warcraft III questions tonight. The total amount of units you can control depends on the leadership of all your heroes. For example, if you have one hero with a leadership of eight, that hero could control eight units. Old information that was released long ago in the first previews.
Different heroes have different leadership abilities.
It is estimated that the hero with the best leadership ability will cap at 12. Currently the number of heroes you can control at once is four. However, as with all things, this is subject to change as we balance the game. New information here, as previously we have only seen the player controlling one or two heroes. So if you can control four heroes, with each hero maxed out on leadership, averaging to about 6 levels for each of them, that means you will be able to control approximately 24 units.