Like the title says, how do I remove x number of random items from the player's inventory?

Use SKSE to loop over the forms in the inventory and put them in a formlist.

randomly pick a slot in the formlist.

use the form in that slot to remove the item from the player's inventory

repeat a random number of times

I agree with DocClox on SKSE use, not on the method.

Try to make the player reference an object reference and use http://www.creationkit.com/GetNumItems_-_ObjectReference SKSE function, make a random number based on the index gained and use http://www.creationkit.com/ObjectReference_Script to point at the item and GetItemCount...and then a random number between 1 and the number of items to define how many items remove.

Agreed. That's much better.

Thanks everybody. The help is greatly appreciated. However, I have one, uhm, probably silly question, but how do I compile the Papyrus script with SKSE functions/methods? I always get a compile error in CK...

Have you downloaded and installed SKSE?

There are a pile of amended scripts that need to be copied into your data/scripts folder. It should happen as part of the normal install process.