It depends on how you plant the HC. Its a standard two inch pot, and the HC is on average another inch bigger than the pot, so you are looking at about three square inches per pot. Some people take each pot and divide it into several little clumps and plant each clump about an inch apart and wait for it to fill in. How fast it does this will depend on how bright your light is, C02 level, and nitrate levels. A fertile substrate helps as well. Jeff Senske has planted several aquascapes with potted HC. He breaks each pot into several smallr bunches leaving some of the rockwool still attached. On one scape he describes here using I think 20 pots? You might want to ask him for details.
As far as fast growing stem plants just to soak up nutrients
My favorites are Myriopyllum, egeria najas, Moneywort, Rotala indica, ludwigia repens, and any hygrophila specie. Hygro corymbosa is a large stem plant, and soaks up nitrate real quick