Depends on the object size, hardware, if it is in the background or not, etc. around 10k for character or more for next-gen.For game models, this is usually from 1024x1024 to 4096x4096 or more for next-gen.Or how the target hardware GPU memory lets you use it. for textures on low poly you want them 2x larger than a texture that has the same size pixels on a model as the pixels on the screen.The appropriate level of detail depends on how small the object will be on screen - basically how many pixels it will cover on screen: That means that for mobile phones this is different than for workstations. What is considered low-poly and high-poly changes with time and computer power, it's safe to say that low-poly is everything that can be displayed on target hardware in real-time. The details from high-poly can be baked into texture maps. You convert high-poly to low-poly through retopology, or decimation.
You convert low-poly to high-poly through sculpting, or Catmull-Clark subdivision.That's why you see so much about low-poly - with knowledge about low-poly topology the high-poly comes along. The problem with low-poly is topology, which has to follow some rules, so the shading looks right, the subdivision surfaces are right or the sculpts will be right. low-polygon proxy geometries of high-poly ones - for rigging and animating.generated by some simulation, 3D scanning, etc.through control low-poly cages (subdivision surfaces).The problem is how you control such an enormous amount of vertices: Highpoly is important to achieve superb surface detail.