CMS licenses for website developers

I’m a website developer but only ever really done work for the companies I work for. I have recently thought about building websites on the side for clients. If I use a CMS, do I need to find a CMS that allows developers to charge to build websites on that CMS or does the CMS need to offer some sort of license to do that?

If anyone has any similar experience with building websites on CMS’ for clients I would appreciate your input 🖥