It is critical that the supplier keep customers apprised of...
It has nothing to do with singular or plural. As written, this is a mandative subjunctive construction as evidenced by the use of the plain verb-form "keep".
There's no reason why you can't drop the subjunctive and use the non-mandative declarative with the tensed form "keeps" -- the only difference is one of formality, with the subjunctive being the more formal option.
Incidentally, 'subjunctive' is NOT a mood form but a construction type recognisable by the use of the plain form of the verb.
And it's NOT a directive either, though it does invoke the concept of 'compliance'.