Any modern router supports WDS, which is used to link multiple routers together wirelessly. you can use extra switches, preferably hooked directly up to the switch ports of your router. the modem should be hooked up to the internet port of your router since the modem can only assign 1 IP address (usually it is DHCP but sometimes it is PPPoE for things like DSL). routers typically cannot handle more than 10 devices.
see the wikipedia article on WDS or your router's manual on WDS.