Where you are using static or default routes then you can only create a single default route, though you can create multiple static routes via each subnet. For instance, if you have pool members in the range 10.2.0.0/16 via vlan 123 then you can add that route but you an't have multiple default routes. Of course you can use different route domains so that you have separated networks and routing, in which case you have a route per RD. You can also use dynamic routing if you want to have more complex routing setup without having to maintain static routes. You can also set the nexthop via iRule.
Regarding incoming traffic, autolasthop means that it will always go back to the device from which it arrived so you won't need those routes. It is common to just have a default route to enable the BIG-IP to be able to get to the pool members.