Ihr glaubt ja gar nicht, was ich mir für Sorgen um Euch gemacht habe!
How does this construction compare to:
sich um etw. Sorgen machen
I'm puzzled by two small changes:
The "was" has taken the place of the "Sorgen" as the direct object.
The preposition "für" is now placed before "Sorgen".
Personally, I would express the same idea as below, keeping the construction "sich um etw. Sorgen machen":
Ihr glaubt ja gar nicht, wie viele Sorgen ich mir um Euch gemacht habe!
Ihr glaubt ja gar nicht, was ich mir für Sorgen um euch gemacht habe!
Ihr glaubt ja gar nicht, was für Sorgen ich mir um euch gemacht habe!
Ihr glaubt ja gar nicht, welche Sorgen ich mir um euch gemacht habe!
In this example, was für is a fixed phrase meaning welche. But the latter sentences sound a bit ungelenk because of the ich mir um euch pronoun cluster. That's the reason welche had been replaced by was für and that one had been split into two parts.
There's another reason to use was für instead of welcher/welche/welches. It's about asking for a known or unknown thing, or a thing vs. a kind of thing:
Welches Kleid ziehst du an? (which of the ones I know of?)
Was für ein Kleid ziehst du an? (which kind of dress?)
In both uses, was für is a question word of its own, meaning what kind.
