How many UCC SSL Certificates (or Subject Alternative Name Certificate) can we install on our server? We're looking to whitelabel our app such that those rebranding our web app (potentially 100+ domains) could use their own domain name, rather than a subdomain of our domain.
There does not appear to be a limit on SANs per certificate. However, I can't say that issuers won't have their own limits (you'll have to do this research yourself, SF does not give shopping recommendations).
Are you planning on reissuing the same certificate, every time you want to add a Alternative Name to it? Unless you have an API with your issuer, this could become cumbersome to automate since you can't just "append" a SAN yourself, whenever you like.
The other issue is that if someone inspects the certificate, not only does it give a list of all the other clients that have whitelabelled, it'll give your details. Doesn't seem very whitelabel to me.
If you are asking, is there a limit on the number of different SSLs there can be - then no, there is no limit (within other resource limits). You will, however, need one unique IP per SSL certificate.