You can create as many routers as you need. Here is an example configuration for the scenario when you need to have per-tenant routers: http://docs.openstack.org/grizzly/openstack-network/admin/content/app_demo_routers_with_private_networks.html

Not sure about external networks, but I think you can create multiple external networks if you have spare NICs on your server, because each external network requires one physical interface.