Depends on the size of the army. But it will only remain a fraction of the army size. And if we’re talking knights, that means a feudal system like what we saw in the medieval period.

Now, armies in that period were rather small. This was due to the limited capacity of a decentralized state with little authority to raise, pay, feed, and maintain large armies. You need a large and complex bureaucracy for that, and the medieval period is characterized by a lack of such a bureaucracy. That’s why kings have land to lords, who gave land to their lords, who gave land to their lords, who gave land to minor lords and knights, who maintained a class of peasantry…

So armies are kinda small. The battle of nicopolis? A massive throw down between the well oiled military machine of the Ottoman Empire and a Holy Alliance of christian powers? 40 thousand soldiers tops.

The battle of Agincourt? The most powerful kingdom of medieval Europe against a rising warrior king?

35 thousand tops, with the French having maybe 25 thousand at best, with 10 thousand of them being men at arms. The loss of six thousand of such men crippled the kingdom’s fighting capacity.

So maybe 1/5 to 1/3 of your army is mounted, and maybe 1/2 to all of them may afford heavy armor.

This stuff was expensive