For a prompt de-icing, you really need a pair of de-icing rigs working a single aircraft. Often the case at those airports where icing is prevalent.
Airport took control of handling equipment a few years ago, with handling agent staff now using a common set of equipment on each stand. Seems to have worked well.
De-icing will be slightly more specialised with the rigs only available for aircraft washing in the Summer months and the training of suitable staff more critical.
I think a minimum of 5 rigs would be necessary for an efficient operation. One being replenished while the other 4 are working.