Certificate limit is 12 certificates for development and 3 certificates for production for $99 developer account.
Similarly I can create any number of development certificate for $299 enterprise account. I have 24 developer certificates and I can still create more. However, I can create only 3 production certificates in my enterprise account.
In case you still want to create a certificate if you exceeded the limit, you have 2 options.
Revoke one useless certificate and re-create 1.
Export cer from keychain of 1 mac and generate .p12 file and copy it to another mac machine and add it in your another mac's keychain. That way you need not revoke a certificate and you can re-use .p12 certificate file.
