Tango requirement (3)
I believe the answer is:
two
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tango requirement</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for two that I've seen before include "prime -- that's not odd" , "Minimal number of partners" , "Number of faces of the god Janus" , "limited company?" , "A low number" .)