Greatest possible quantity (7)
I believe the answer is:
maximum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greatest possible quantity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for maximum that I've seen before include "Highest amount" , "Greatest possible quantity" , "Utmost" , "Largest in group of numbers" , "Upper limit" .)