When a T0 or VRF is created, the system will automatically create a number of prefix-lists for each instance.
These prefix-lists are required for route redistribution.
This explains why there may be a large number of system created prefix-lists present even before user created ones are added.
The capacity dashboard alert incorrectly bases its calculation on a global prefix-limit of 500.
The limit is in fact 500 per T0.
Note this is a soft limit and will not prevent the creation of up 100 VRFs per T0.