We look at how a star evolves by looking at how its temperature and luminosity change as it gets older. Like the H-R diagram but rather than plotting how the star looks now, we plot its lifeline or evolutionary track. Basically similar but some differences in tracks, again tied to mass. Can look at a massive-star forming region like the Orion nebula and see the different masses of stars forming and build up the evolutionary tracks. Stars never form by themselves.
Molecular clouds are thousands to millions of solar masses of gas and dust, and can form many stars.
10 stars with masses between 2 and 10 solar masses 50 stars with masses between 0.5 and 2 Several hundred stars with masses less than 0.5 solar masses. There are limits to the most massive and least massive star it is possible to form. Theoretically, if you tried to form a star more massive than 100 solar masses the energy generation when hydrogen burning started would tear the star apart, and gravity couldn't hold it together. We don't see stars that massive, even though we should if they existed. The remnants will last as long as the Universe.