IP address is an unique string of numbers which identify individual devices on the internet. IP addresses are used by computers to identify each other as well as to locate resources or services on the internet.
The number of IP addresses needed depends on the size of your network. If you have only a few devices, you may need one IP address per device.
For more complex networks that include multiple computers, each running multiple services, additional IP addresses will be required to separate these services.
Many routers support port forwarding as well, which means a single public IP address can be used to provide access to many services on private networks behind that router.