The number slider controls the number of termites. The density slider controls the initial density of wood chips. That is, termites sometimes take chips away from existing piles. That strategy might seem counter-productive. In general, the number of piles decreases with time.
Some piles disappear, when termites carry away all of the chips.
And there is no way to start a new pile from scratch, since termites always put their wood chips near other wood chips. So the number of piles must decrease over time. There is no termite in charge, and no special pre-designated site for the piles. Each termite follows a set of simple rules, but the colony as a whole accomplishes a rather sophisticated task. They are just represented as colors in the patches.