Cisco switches allow you to set an unlimited amount of secondary IP addresses per interface. To set an IP address for the interface, use: ip address <IP> <mask>
^^that is for the first IP address.
For additional IP Addresses, use: ip address <IP> <mask> secondary
That really depends on what you’re trying to accomplish: If you have 3 VLANs that you want to connect (trunk) through the router, then yes, subinterfaces on, say, fa0/0 is a great way to go..
if you just need a secondary IP address on the interface, Ryans answer is it…