So, alter self allows you to turn into any 5HD-or-less member of your type, be it humanoid, outsider or vermin - but HOW MANY times you can change your form per cast? As far as I've got it, it's any amount of changes and tweaks in the duration of the spell, thou some people say it's not. Is there an official limitation about it?
When you cast the spell, you change form to the desired form. That form remains until it ends, is dispelled, or is dismissed. It gives no ability to switch between forms in a single casting.
The precise text:
That is the effect of the spell when it is cast. Spell effects take place when cast. Compare to shapechange, which does allow you to change repeatedly in one casting:
Here the spell is saying that part of its effect is to grant you this ability to continue to change. Alter self does not give any such ability, it just grants the desired change chosen at the time of casting.