Starcraft – Do I need two licenses to play StarCraft 2 multiplayer over a LAN

Judging from your question you want SC2 over LAN without connecting via the Internet. There is no easy way to do this. As McKay points out, as long as all computers are connected to the Internet you can use computers which happen to be connected to the same LAN, but make no mistake you will not be leveraging the advantage of said LAN.

Now I said "easy way," because there is a "hard way." Blizzard does have a LAN client of Starcraft II which is provides for reputable tournaments. Gaining access to this requires setting yourself up as a business organization and paying a licensing fee. For more information you're going to have to contact Blizzard directly.

That said, for a couple of friends, there is no legal way to leverage your LAN for SC2 games without going through Blizzard.

Regarding AI vs Real Humans, you'll never get better experience than playing against human beings. That being said, the AI can be used as a tool to gauge your Build Order, dealing with timing attacks and executing counters.

But your ability to shut-down the AI is not a good way to compare yourself to other players. So being able to defeat Very Hard doesn't mean you're about gold level play.

If I had to break my playing against AI vs against Players, I would say about 10% is AI and the rest is playing against real players. And the replays against humans are the ones I really pay attention too because those will teach you the most.