If you have a certificate with currently 61 sub-domains on it, and you have a new customer (xyz) then you would need to “expand” the certificate to include xyz. The certificate would then be valid for 62 sub-domains.
In reality, this is a new certificate with all 62 names on it, so it will be valid for all the original customers as well as the new customer.
If you have large numbers, it may be that a purchased wild-card certificate is the best option for you. can you provide a little more background to your setup, the numbers involved, how frequently they change etc ?