Just to make sure things are clear...
The main limit is Certificates per Registered Domain (50 per week). A registered domain is, generally speaking, the part of the domain you purchased from your domain name registrar. For instance, in the name
www.example.com, the registered domain isexample.com. Innew.blog.example.co.uk, the registered domain isexample.co.uk. We use the Public Suffix List to calculate the registered domain. Exceeding the Certificates Per Registered Domain limit is reported with the error messagetoo many certificates already issued, possibly with additional details.
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Renewals are treated specially: they don’t count against your Certificates per Registered Domain limit, but they are subject to a Duplicate Certificate limit of 5 per week. Note: renewals used to count against your Certificates per Registered Domain limit until March 2019, but they don’t anymore. Exceeding the Duplicate Certificate limit is reported with the error message
too many certificates already issued for exact set of domains.