JAR OPS 1.865 says that you must have one VOR, one DME and one ADF as a minimum (except that you don't need an ADF if you won't be using one at any stage of your intended route).
Paragraph (c) part (v) then says that you need an additional ADF (to make two) when navigation is based solely on NDB signals. That would mean that for an NDB/DME you would need just one ADF and one DME (as DME is considered as a means of navigating) but for a pure NDB approach with no other navaid available, you need two ADF's.