Hi. If i get unity 2.6 pro. and they release others version later (2.7 /2.8/3.0) I'll have to buy another license?
When you buy Unity 2.x, all further 2.x updates are free.
Major updates (2.x -> 3.x etc.) are paid. We have only done that once (Unity 1.x to 2.0), and the upgrade price was about 45% of the normal price (Unity Pro: $1500, upgrade from 1.x Pro: $650). We did a free upgrade for folks who bought Unity 1.x some months before 2.0 was out.
The exact pricing and structure for Unity 3.0 upgrade is not determined yet, but probably it will be something along the same lines.
Historically, Unity hasn't charged for minor version updates, even though some of those updates have added major features (e.g. Unity 2.5 added a Windows version, and Unity 2.6 added external version control compatibility). I don't know what will happen with Unity 3.0 but if I recall correctly, there were some upgrade inducements during the Unity 1.x to 2.x transition.