Synchronized collections from Lightroom Classic are smart previews, and do not impact the 20 GB limit. If you have 20 GB used then it is because you have downloaded images directly to Lightroom for the cloud. If those images have synchronized back to Lightroom Classic then you can safely delete them from Lightroom for the cloud to free up that space. If those images have not synchronized to Lightroom Classic, and you delete them, then they will be lost. Not sure what your workflow is so take time to consider things before you do something you might regret.