What makes a model Low-Poly?

Up to 100K polygons, maybe a little more but less the better.
No standards and faceted appearance is a stylization and not a condition.
Low poly models can look smooth like a high poly.

For Vehicules, 200-400K polygons are sometimes considered Low poly. For some basic objects like mods for games (GTA 5, MSFS or roblox etc..), 10-30K are usually low poly models. So mostly, each object has a different range (poly count) in which it can be considered as low poly.