Here is the renowned foundation for responsive design:https://abookapart.com/products/responsive-web-design
Here is the good free course:https://www.udacity.com/course/responsive-web-design-fundamentals--ud893
In short these guys say your breakpoints really depend on how you want your site's design to look when resizing the browser window (in Chrome dev tools responsive mode). E.g. if at some width (or height) the particular part of your page starts looking out of order, apply breakpoint here with necessary CSS changes. You can have as many BPs as you see fit.
For each different site your BPs can be pretty different. This is normal.
LESS does not work in browser. So irrespectively whether you are in development stage or in the production one, you have to compile LESS to CSS and call the CSS files from your HTML. Usually the compilation is done on the fly using task runner (say Gulp) and the respective LESS compilator plugin.
To my opinion it should not matter for you if LESS is widely used or not. In broader sense if you ever plan to engage CSS in your web development, you will inevitably come to using LESS (or SASS/SCSS) instead. Make an experiment. Write 20-30 lines of CSS for some of your site. Then write the same in LESS. Your senses will tell you why use LESS.