When I was first testing out my automation I was using real keys instead of the staging server, so every time I tested I was eating up a cert.
In the end though, I have more than 7 certs for my main domain--With a bunch of services built up over the years, I have roughly 14 or 15 certs (so it took me 3 weeks to get all my certs created). Subdomains count against your domain limit, so "mail.example.com", "blog.example.com", and "www.example.com" count as 3 certs for "example.com".
Are you 100% sure subdomains count against your limit? I just asked that question on letsencrypt recently and was told there is no limit to subdomains.
In the end though, I have more than 7 certs for my main domain--With a bunch of services built up over the years, I have roughly 14 or 15 certs (so it took me 3 weeks to get all my certs created). Subdomains count against your domain limit, so "mail.example.com", "blog.example.com", and "www.example.com" count as 3 certs for "example.com".