https://www.statista.com/statistics/259396/global-combat-hel...
Even if the numbers lost aren't on their own crippling, this will also force Russia to either relocate its helicopter forces further from the front lines (reducing the overall utility of its helicopter fleet) or invest substantial resources into better defending its bases in the area (of course coming at the expense of defending other critical locations).
So as some other commentators have pointed out, 21 isn't 'staggering' in the scheme of plausible total fleet size (though we don't know what the mix of models destroyed was was - obviously Ka-52s losses are far more impactful than Mi-8 losses), but the size in a single strike will force adaption.
Finally in absolute terms - 21 is just a big deal period. 21 choppers is on the order of a US army aviation battalion. The most disastrous modern attack helicopter operation by the US Army (the attack on Karbala in 2003) deployed 31 AH-64s, suffered one shoot down, one downing due to equipment failure, and damage on all 29 other airframes (it took about a month for the formation to repair and reconstitute and be able to deploy again). Karbala forced changes to helicopter deployment by the US army, and we'll likely see similar forced adaption by the Russians here.
The Russian army has been strikingly bad at learning. ATACMs’ range is known. That they were being delivered to Ukraine was known. Yet here are closely-spaced places and choppers within firing range.
Same for e.g. doing in-person meetings of senior leaders in Sevastopol, or recapitulating Bakhmut in Avidiivka.
The evidence doesn’t sustain this hypothesis. Within a single command line, lessons are learned. But the effectiveness of tactics moves across the battlefield after demonstrated success is nuts.