Creative cloud storage.

The number of photos you can store with 20GB depends on what type of files they are. What camera do you have, and are you shooting raw? 

There are also other factors that can be filling up the storage. First, images that you delete don’t get immediately deleted anymore. They go to a deleted collection for a period of time. Check the deleted collection and delete anything that is in it to permanently delete.

Files saved to Creative Cloud Files, Creative Cloud Libraries, and Mobile Apps (Photoshop Mix, Fix, Sketch and Draw) all count against your 20GB of storage. It’s possible that one of these is filling up your space.

If you want to manage your photo library with Lightroom, and not fill up your cloud storage you should consider switching to Lightroom Classic. There is a bit of a learning curve to set it up correctly, but it works great when you do it right. Classic manages images on local storage drives and syncs to the cloud by way of smaller smart previews. You can selectively sync collections instead of putting all of your photos in the cloud.