Last updated on December 26, 2018
By looking at <a href="https://www.ietf.org/rfc/rfc3768.txt%20RFC3768" rel="nofollow">RFC 3768</a>, section 5.3.9 seems to indicate that indeed you should be able to configure more than one virtual IP in a single instance:
What would you call a manufacturer that allows you to define secondary IPs on an interface/VLAN, but not in a VRRP instance on that interface? Instead, they say you should just create another instance. While that would work, interoperability with other gear may be at stake. We're talking about Fortinet, and all I ever hear from them is "our gear just works that way".
5.3.9. IP Address(es) One or more IP addresses that are associated with the virtual router. The number of addresses included is specified in the "Count IP Addrs" field. These fields are used for troubleshooting misconfigured routers.