Hi,
i assume it's GAIA, what version?
do you want to have each default route with different priorities ? what is your goal?
where do you see only two? with 'show route' clish commands, or 'route -n' (expert) you will see the selected active route only, with 'show route all' you will see all routes also the inactive ones.
if your goal is legacy redundancy with routes, you can configure different routes with different priorities. and you also can set some probing target or ping to validate the nexthop responsiveness
if your goal is more advanced like load sharing between ISPs, you may use our Quantum SD-WAN, or ISP Redundancy feature. based on your needs,.