Just in case anyone's still looking, here are the details:
For every additional 8 rules you want to apply, you create a new "dummy" zone... that is, a zone that is not used for anything, like so:
set security zones security-zone dummy1
set security zones security-zone dummy2
set security zones security-zone dummyN
...
Then, you can create rule-sets using your source zone and a dummy zone, like so:
set security nat destination rule-set DestinationNAT1 from zone dummy1
set security nat destination rule-set DestinationNAT1 from zone untrust
set security nat destination rule-set DestinationNAT1 from zone dummy2
set security nat destination rule-set DestinationNAT1 from zone untrust
...
So that each rule-set has a unique source (untrust or dummy1, untrust or dummy2, etc...) but in reality it's just going to match untrust since the dummy zones aren't used.
Hope this helps someone!