Is it 5 per 7 days? This could (with some useless efford to choose the correct days) help for a moderate number of domains. I thought it is 5 per domain and 60 days, with up to 100 names on a cert.
I still vote for a global limit. Give us 200 certs per 60 days. If they are for the same domain, for subdomains or for 200 domains should not matter.
And grant some bonus to first time domains, so there are some failed attempts granted. Renewal should then need less certs for people, who for example forgot the www. subdomain on the first try of a certificate.