This issue occurs due to an overflow. The game allocates 2 bytes for the stack size, so the maximum number of creatures in a single stack should not exceed 32767 (if the most significant bit represents the sign) or 65535. When more creatures are added, an overflow occurs, and the value wraps around to 0, resulting in negative numbers. I (and others) have explained this in detail in a specific thread. The same can happen with experience and resources if you play on the same map for a very long time and excessively level up a hero or accumulate resources.
By the way, what map are you playing on? And in which version of the game do you have 80,000 liches? It's most likely WoG, as it has additional features that allow you to quickly gather armies.