Numbers, yes, but also quality. The T-34 when it appeared shocked the Germans. While it had significant flaws the basic tank characteristics like armor, firepower and mobility were very good for the time.
The Germans thought so highly of the T-34 that they thought of cloning it, but came to the conclusion that designing a new tank to counter it was in the end a more feasible idea. This process eventually resulted in the Panther, often thought to be the best tank of the entire war. Although in practice it was beset by reliability issues, and of course it was too little too late in general by then.
Getting back to the original topic, the lesson seems that for tank vs tank combat, large numbers of good enough tanks win the day. Now, the T-72 did disastrously poorly in the Gulf war, but OTOH apparently 'domestic' USSR T-72's have better armor to begin with than the export versions used in Iraq, and further upgraded ones with more armor and modern sensors might be a much harder nut to crack.