Currently the maximum amount of selected units is 16. If you need to see how many units are tasked to what resources go to the world map and select your city. Then inside the bottom left corner of the UI select the hammer to see your peasant units. They will be sorted by tasking's. If you need to find the ones that are not doing anything create an army with the idle units and then send them back into town. They will be at the entrance of your city awaiting orders. Hope this helps.
If I always appear prepared, it is because before entering on an undertaking, I have meditated for long and foreseen what may occur.
--Napoleon Bonaparte, 1831