I am building a player model for my game, the model is very complicated due to its ability to be customizable. A single player entity can have 70 + entities including the head, parts to the face, arms, legs, shoes, and the clothes and armor it wears, and the weapons it has equipped. I just need to know, at what point does the unity game engine start lagging due to too many entities? Also, the game is low poly, there are many entities, but each one is block based, or very simple in design.
You can't generalize that because the resources consumed by an entity (CPU, system-memory, bus bandwidth, GPU and GPU-memory) vary greatly depending on their components. Do they have colliders? Do they have renderers, and if yes how complex are their meshes and their shaders? Do they have scripts? Do they have rigidbodies? Are they animated? All that (and many more) affects how much an entity taxes the engine.
Also, different target platforms have different resource capabilities. When you are targeting smartphones you won't be able to do as much as you would be able when you are targeting gaming PCs.
But keep in mind that you might be able to reduce the number of game objects by modifying meshes at runtime (which is useful for things like face customization) and merging meshes (which allows you to combine multiple objects into one object with one mesh).