One way, that you alluded too is to do a second pass on at least that
part of the document, using xx:node-set() then you can just use count().

If that is not a suitable method, the elements generared must be a
function of the input documents, so in principle you should be able to
work out the number that are generated as a function of teh same input,
but without knowing any details of your transformation it's impossible
to say what that function is.

David