In versions 6.0.x and previous, the maximum number of usable LDAP servers you could define was ten. You could define more than that, but the Fortigate would only check ten of the defined servers (and not in a predictable way, it appeared pseudo-random).
I have a doc from 6.0.x that indicates this limit:
https://docs.fortinet.com/document/fortigate/6.0.0/cli-reference/15405/user-ldap but I have not been able to find a similar one for 6.4.x.
Has the limit changed?