Is there a benefit to having more then 1 academy ?

Yes, for every academy you can have 10 more troops on the field. And by troops, I mean stacks of 10, so altogether an extra 100 guys. You'll be told you can't produce more at this time if you go over your allotment.

Click "Military" on the control panel of your map page to see how close you are to running out of room, you'll see something like 28/30 near the top of the page. This means you have only two available spaces left to fill before you'll need another academy.

They can also be used as an expensive "gate" to keep enemy troops out of your castle or other valuable spot.

I was wondering how to be able to make more armies, I thought I needed to own more castles or something.