[joshmoz]

It's also worth pointing out that you can have up to 100 SANs in a certificate, so you can technically issue for up to 20x100 (2000) subdomains per week.

[shawabawa3]

When I hit a rate limit on sub1.example.com, it let me create a certificate with a SAN for sub1.example.com and sub2.example.com

So I think it's actually essentially unlimited, you just can't request the exact same certificate more than 20 times per week

[nsgi]

There are separate rate limits for FQDNs and domain names. The former is 5, the latter is 20.