Well if you look at the pic of the VRAD in this thread it'll give you an idea. Basically it's gonna be a multiple of 24. The one in the pic is capable of 396 ports but only has cards for 144 installed.
is it possible to know how many subscribers are on your VRAD? I'm having an intermittent problem with QOS that affects my VOIP phone. One theory is excessive number of subscribers on the node or all of the subscribers simultaneously using the internet, causing a short drop in service