Aerojet, the company that reworks the old engines before sending them to Orbital, purchased 36 engines. That's enough for 18 Antares rockets. That will cover the current CRS contract and maybe a follow-up or some commercial satellite launches. Beyond that Aerojet has a license to manufacture US-made copies of the engines. That isn't as easy as following a recipe, but it is something they are working on.

Orbital has the US rights to build new copies of the original NK-33. Presumably they'd set up some kind of production line once they have enough customers to exhaust their russian-sourced supply.