Some have left out a few things. In the past and still many ISPs will only give out groups of 7 or 8 IPs. Since the first IP is the network ID and the last is the broadcast address, you are liable left with five. Given the shortage of IpV4 addresses, ISPs started to give out 1 IP. It isn't always, "I'll take three please." Your subnet on the public side will usually be 255.255.255.248. Subtract this from 255.255.255.255 + 1 (first number counts), and that is how many IPs which make up the subnet. Deduct two and that is how many usable IPs you have.
It would be good to check with your ISP as to bundles and pricing. I started with one, then added a VoIP, then added an isolated wireless network, and I am down to two. Yes, you can use a VLAN and yes you can use the guest on the wireless, but I prefer it this way.
