Can an AD service account host SPNs for multiple SQL Instances?
Yes it can. Eventually If you register too much spns with the same maybe you can fill the ad attribute. SPN is a non-linked-value attribute, and so you run the risk of overrunning the hard-coded 8K limitation within the underlying ESE database.
What you can't do is to share one spn registration with two service accounts.
My suggestion is to take advantage of this tool:
Microsoft® Kerberos Configuration Manager
https://www.microsoft.com/en-us/download/details.aspx?id=39046
Run it (as admin) and it will generate the proper setspn command for kerberos setup.
Eventually you have to register your instances both with netbios &amp; fqdn names.
And please remember that you can register an spn on a domain user account or on a domain coumputer account.