I'm planning to set up 5-8 routerOS metarouters in a 493UAH for building up a big PPPoE "dialer": each metarouter just dial pppoe, NAT the outgoing traffic and send it to its parent router using the vif bridge.

I have a doubt about RouterOS License.

RB493 comes with Level 4 license.
What I want to set up inside the 493 are N Level 4 RouterOS.

Do I have to buy a license for each metarouter I setup inside my box?
How is licensing going when metarouters comes into scene?

You inherit licenses from your parent router.

What I want to set up inside the 493 are N Level 4 RouterOS.

And?

Metarouter are virtual routers running on top of Mikrotik's RouterOS. This can be interesting when you want to use both RouterOS features and OpenWrt features in the same box. It also is a way to avoid having to worry about recovery of the RouterOS installation and the attached license