I think I read here that 400-450 is the minimum number for most 3rd Rail models in the $650-$750 range and that is combined 2-rail and 3-rail reservations. Twenty units is required for a specific road name to be done.
It seems as if 3-railers are OK with the inferior SD40-2's from Lionel and MTH and that is one reason why reservations are moving along at such a slow pace. It's too bad that companies like 3rd Rail and Atlas have to rely so heavily on 3-rail preorders in order for a 2-rail version to be offered.