Yes, it’s possible to make certificates for 150 (or some other number) of subdomains. You would only have to validate the main domain if you wanted a certificate for the main domain.
Since a certificate can include 100 names, you can do this with two certificates, so the rate limits would be no problem.
However, if you want to add and remove subdomains frequently – i.e. more than 20 times per week – that would run into rate limit problems.