The Web Analytics FAQ indicates that there is a 10 site soft limit, and to contact support to increase the limit. However, seeing as I’m on the free plan, I don’t seem to be able to contact support.
My hunch is that they would not approve a limit increase on the free plan anyway.
However, the 10-site limit is for sites not proxied through Cloudflare, where you have to put the beacon script in your HTML yourself. For sites that are proxied through Cloudflare, there’s no limit and they shouldn’t count against your 10, so you could still have some non-proxied sites. You should also be able to free up space by proxying sites that are not currently proxied.