What about breakpoints?

• Pay attention to design, not devices (there are simply too many)
• Start with a single breakpoint; introduce others only as needed.
  Put another way, you don't have to have a half dozen different layouts just because you see some definition lists of XS, S, M, L, XL, XXL sizes defined.

With that said....

Breakpoints defined in certain approaches are:

• 576px, 768px, 992px, 1200px, and 1400px (Bootstrap)
• 600px, 960px, 1264px, and 1904px (Material Design)