I didn't know that you can use a number+counter combination as a noun, but it seems from this sentence that you can.
She picked up the sheet that had been placed at the top
Is this always true and can it be extended to numbers other than one, e.g.
She picked up the top two sheets
What is the alternative? i.e. what would I do if there was no counter word (other than a generic counter) and I hadn't previously introduced the thing I was talking about e.g.
Please pass me the top two plates from that shelf.
I can't use a generic counter because there are also other things on the shelf and the person wouldn't know which thing I wanted.