If i was to connect 2 Lans via the internet using NAT. Would i need port mapping?? or just address translation?

you can't connect two lans via the internet using NAT assuming you are using private ip. Might want to google VPN [virtual private network] Give a person a fish, they eat for a day. Suggest they internet search and they learn a skill for a lifetime.

Why cant you connect 2 LANS via the internet using NAT? Is it because there will only be 1 IP Address going out of each network?

I would agree with the first reply based on what I first read. It depends on what you wish to use on the lans. You'd have to give us more detail on what services or files or applications you wish to share across the internet. If you have two IPs that are public and wish to access the other lan that is fine. You are correct in that there will be 1 ip on each section. The reason for NAT or more clearly PAT (since you wish ports to be assigned not just the network address) is so you can translate the Public IP to one or more private IP address. That is not always an easy task. Some applications are not very internet aware. They use network ports that are dynamic both in start and in use. Some applications use well known ports and can be mapped. Remember that what you wish must have some sort of routeable means and that normally involves tcp/ip in this case. To get some services to work you MUST use a VPN or other tunnel process. Consider too that you might require some sort of security on that system.