If one assumes that there were about 200,000 active personnel in the Ukrainian army, and of these about half were in combat units (which is a pretty extreme tooth-tail ratio), 50% casualties, or 50,000 men, would be in the ballpark after six months of war.
Consider the equivalent Russian/L&D casualties as a % of the force originally committed.
Ukraine could replace its casualties from reservists and new conscripts, and they seem to have done so. The country seems to have lost two brigades outright (those at Mariupol). But it rebuilt others, such as the 80th, crippled at Kherson in the initial attack; it was one of the lead mech brigades in the Kharkiv operation. And it created a bunch more. There are at least 2-2.5X as many brigades now.
a NATO army manned by Ukrainians