When configuring Discovery rules, I was able to at least add 12 checks.
However, the server doesn't seem to handle more than 8 checks.
This is the maximum amount of checks I see in the monitoring screen for the discovery.

Also, the event monitoring does not list anything for the checks that don't show up.
Subsequently, actions using these checks are not performed.

It seems to me a hard limit of 8 checks per discovery rule is defined.
If that is the case, the frontend should limit the amount of checks that can be defined in a rule.
Otherwise the limit should be removed or raised to the amount that is defined in the frontend (if any).