I have asked about this and it turns out that this is indeed a hard limit that’s intentionally set up on our platform.
There are not that many legitimate use cases to have this many domains, without also running into other limits (CPU, hits, inodes) first.
I will say that I was not aware that such a limit existed. Very few people have this many subdomains, and so far you’re one of the first to reach out about this. I personally don’t support presenting something as unlimited when there is clearly a hard limit on it. But there are limits on accounts that I also don’t know about.
In any case, this limit is by design. That means that you cannot create more than 50 subdomains on your account.