I assume you didn't configure this. Then if you found out that many auth rules are configured, you need to understand why they were created at the first place. Then I would recommend clean them up first (probably that's what you're trying now).
To do that, you need to take look at inside of those rules, or just "show" under "config vpn ssl settings" to dump all config including auth rules. They might have different interface like in the KB, or have different realms. Below is an example from one of our FGTs we use for SSL VPN with a realm after some modifications.
xxx-fg2 (settings) # show
config vpn ssl settings
set source-interface "Internet"
set default-portal "g2-users"
set groups "user-g1"
set portal "g1-users"
set realm "g1"
set groups "user-g2"
set portal "g2-users"
As you can see the first auth rule is for "g1" realm, and the second one is without realm. Then neighbor of them have source-interface specified. Means both use "Internet" zone configured directly under settings. Also pay attention to the "groups" configured in each auth rule. This is the key you can use to search in policy if they're actually used like below:
la-fg2 (policy) # show | grep -f user-g2
config firewall policy
set srcintf "ssl.root"
set dstintf "InternalNetworks"
set srcaddr "all"
set dstaddr "Internal-routes"
set groups "user-g2" <--- the group used
Then you now know the policy 6 is using the second auth-rule without realm.
In your case, I would just change source-interface to "any" wherever in the settings that has an interface configured.