Equal-Cost Multi-Path (ECMP) Routing

The ASA supports Equal-Cost Multi-Path (ECMP) routing.

You can have up to 8 equal cost static or dynamic routes per interface. For example, you can configure multiple default routes on the outside interface that specify different gateways.

route outside 0 0 10.1.1.2
route outside 0 0 10.1.1.3
route outside 0 0 10.1.1.4

In this case, traffic is load-balanced on the outside interface between 10.1.1.2, 10.1.1.3, and 10.1.1.4. Traffic is distributed among the specified gateways based on an algorithm that hashes the source and destination IP addresses, incoming interface, protocol, source and destination ports.
ECMP Across Multiple Interfaces Using Traffic Zones

If you configure traffic zones to contain a group of interfaces, you can have up to 8 equal cost static or dynamic routes across up to 8 interfaces within each zone. For example, you can configure multiple default routes across three interfaces in the zone:

route outside1 0 0 10.1.1.2
route outside2 0 0 10.2.1.2
route outside3 0 0 10.3.1.2

Similarly, your dynamic routing protocol can automatically configure equal cost routes. The ASA load-balances traffic across the interfaces with a more robust load balancing mechanism.

When a route is lost, the device seamlessly moves the flow to a different route.

CLI Book 1: Cisco ASA Series General Operations CLI Configuration Guide, 9.8:
https://www.cisco.com/c/en/us/td/docs/security/asa/asa98/configuration/general/asa-98-general-config/route-overview.html#ID-2101-0000004d