Are you talking small wires or large? For #12 THHN (90C), for instance, Table 310.16 allows 30A, but 240.4(D) restricts #12 to 20A maximum. For derating, though, you would use table 310.16, and would be able to put 7-9 #12 THHN conductors in a conduit before having to derate below 20A.
You do not have to count grounds for derating purposes, and may or may not have to count the neutral, depending what kind of circuit(s) you have. And I don't know if there is a code that explicely states it, but switched conductors (like in a 3-way switch) should only count as one conductor too, as only one will ever be considered a "conductor" at any given time.
