Breakpoints Viewports

It depends on the project. I would use Bootstrap breakpoints as a starting guideline and then adapt to the project content for smooth transitions.

The Standard Bootstrap 3.x Breakpoints

@media all and (max-width: 991px) { ... }
@media all and (max-width: 768px) { ... }
@media all and (max-width: 480px) { ... }

The Standard Bootstrap 4.x Breakpoints

@media all and (max-width: 1199px) { ... } 
@media all and (max-width: 991px) { ... } 
@media all and (max-width: 768px) { ... } 
@media all and (max-width: 575px) { ... }

You need a minimum of 3 breakpoints -  one for each type of device (mobile, tablet, and desktop).

If you know the market for which the site is being developed you can use publicised data to help you decide what breakpoints to target.

Screen Resolution Stats Ireland

Once you have the website live you can use analytics to track the devices visiting it and alter your breakpoints to improve the experience for the most common screen sizes. How deep into the rabbit hole you wish to go is entirely up to you.