Well, the official docs say something different:
Pending Authorizations/Account limits how many times an ACME client can request a domain name be authorized without actually fulfilling on the request itself. This is most commonly encountered when developing ACME clients, and this limit is set to 300 per account week. Please utilize our staging environment if you’re developing an ACME client.
There is no limit to the number of certificates that can be issued to different domains.