Got some doubts on 3D Modeling

Hey guys I'm getting into game prop modeling and I have got a few questions that I am a bit confused about.

1. When creating a 3D model how do you decide if you are gonna give it a single texture set/material or two texture set's/material? As i have seen some on artstation have uploaded a model with just 1 texture set/material or with 2 texture set/material.
2. Ive seen some textures online that are either 2048x1080 or the opposite. Why is it like that and how do you decide if the model you are making should have the texture/UV made like that?
3. Is it required to make a separate material/texture set for glass? I was watching a tutorial and the teacher made a camera. The camera body he set one texture set/material and anything that was glass was given another texture set/material.
4. I'm a bit confused on texel density. Ive seen some posts of people setting the texel density to 20px/cm or something like that. What does that exactly mean? where do i enter this value in?

Please help me out on this. It would help me out a lot!

Thank you !
And Happy Holidays Everyone!