Hello @greggman,
Yes, it is.
If you issue 1 certificate for g1234.user.happyfuntimes.net and 1 certificate for g5456.user.happyfuntimes.net you are using 2 of the 20 certs per domain limit allowed in 7 days.
But if you want, you could add those 100 domains (100 is the limit for domains in the same cert) to the same certificate and it will count just as 1 certificate and not 100, in that way you will avoid the 20 certs per week limit.
Anyway, take a look to the post where Let's Encrypt explains the Rate Limits in use.
Cheers,
sahsanu