It all depends on how the 3D model (scene) is used. And what you as a designer want. This is just a challenge to learn you some aspects of 3D development.
With a low poly model, you have the option to switch on smoothing. Without no extra costs on poly count data. A bevel give you a nice smooth curve. A line on which the vertices of a poly are aligned to. And depending of the bevel detail, it can still be a low poly model.
Low poly can be used to quickly model a scene, to see if the composition of objects, light, scale will fit in your end goal.
When the low poly becomes high poly it takes more processing power.
If you have enough computer power, this isn’t a big issue on the project level most people will work.
But it’s good to start with low poly. Having fast render times, keeping fun with Blender.
From low poly you can create High Poly models, by sub surfacing or sculpting. These high poly models can be used to create textures to be used on low poly models. Textures will then compensate the lack of details on a low poly model, which renders fast.
This is how 3D projects are managed and processed.
For gaming, where low poly count is important, to keep FPS high. You use models in different poly count. A model in the distance you don’t need to be a high poly model, because the details can not be seen. For this Blender has also an option called “Multires”. A game engine can switch between poly resolutions. In this blender course the “LOD” level of detail will be explained.
For 3D printing, it is a different ballgame. 3D printers don’t have a smooth print option. The need as much details information as possible. Depending on your printer quality, you need a high poly model. Then you apply a subsurface modifier. Increasing the poly count.
Same goes for animation. Some Tv Series looks good, but when you watch closely, you will see low poly models, with clever tricks. Poly models for animation, do have different density detaisl of poly because of bending and stretching possibilities (arm pits).
On the other side, high end productions (The lion king) have enough budget to use massive render farms. High poly count, massive textures 8k, etc.
Al these aspects you will learn. And then you can make a decision of the need and usage of your model.
So for some low poly is 20 faces, the other is 1.000. There is no rule, it depends totally on project type, scope and process.
The smooth option, is just an mathematical to let things look smooth even if the don’t have that much (poly) information. But it doesn’t help with 3D printing. Then you still need a high poly count. Or your object will be printed with low poly faces.
