Ukraine has 500,000 troops not counting volunteers and mercenaries.

Ukraine claims it has lost 14,000 troops, but that is likely low. But even at double that, Ukraine is "not running out of troops" and time soon. In fact, given it has 44 million citizens, it should be able to replace losses at that rate indefinitely.

Russia is more likely to run out of troops first. Russia is a far larger country, but it is taking far greater casualties and it can't abandon every border, so its "deployable" force is not much larger than Ukraine's.

Russia deployed 200,000 of their approximately 900,000 active military in its effort in Ukraine. It cannot deploy much more than the original 200,000 and support them logistically. Even that is number is starting to shrink and Ukraine obtains longer range missiles that can strike at the main choke points for supplying its Russian army.

The issue won't be manpower, but trained manpower and Ukraine is developing some battle-hardened veterans, who will soon be equipped with the most up to date weapons in the world (courtesy of the US and EU). Up to now, the army has relied mostly on Russian equipment (since they know how to use it), but by next year more and more of the equipment will be state of the art western equipment.

Putin bit off more than he chew. He is trying to wage a war of terror, but Ukraine does not need its economy to continue the war as long as western nations supply it with weapons and food. Sadly, once the war is over, it will take Ukraine years to rebuild its infrastructure, but its primary business agriculture should bounce back almost overnight.