How can I increase the number of LAN ports on a router ?

In general, you can connect any switch/hub to a router in order to expand the number of Ethernet devices connected to it.

If the hub/switch you want to connect has an Uplink port, all you need is to use normal patch cable, plug one end to the Uplink port (it can be on either device) and the other to any regular port on the other device. Note that if both devices have an Uplink port, you should not connect them to each other !

If neither device has an Uplink port, you most likely need to use a Crossover cable and plug it into any port on both the router and the switch/hub.

Note: Some newer routers have Auto-MDI/MDIX (or auto-crossover) LAN ports, meaning they are capable of automatically detecting what type of device is being connected and work with either.