As regards a single map versus multiple maps.

In a character made for a single purpose, such as a video game character, it is better to make one map.

But for a figure where you may want to change textures, such as the colours of the eyes, or a new face, or make clothing painted directly onto the body, using multiple maps is more logical. Someone making a new face only has to make one map, and you don't have to load a massive 4000x4000 map everytime you want to change something, such as the eye colour.

Another thought. Make your maps square, and in sizes such as 1024x1024, 2048x2048 or 4096x4096. I am not going to go into explaining the mathematics behind this, but trust me that a computer processes and renders a map of 2048x2048 faster than a map of 2000x2000.

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