This limit comes from Let's Encrypt and it's possible you might hit that limit if you have a lot of staging/preview domains.
The workaround at the moment would be to obtain a wildcard SSL for the domain and to install that through the UI on your staging websites. In the future we will have a feature to automatically apply the wildcard certificate to any eligible domains instead of attempting an individual Let's Encrypt request.
A third option, if a lot of your staging websites belong to clients, is to configure the clients as resellers. They can then choose their own staging domain suffix which will come with its own Let's Encrypt rate limit.