As far as I know the thread count of quilting fabric can be different. For Kona Cottons 60 by 60 threads per inch is indicated by the manufacturer - that means a thread count of 120.
There are other fabrics with a higher thread count, for example the Japanese fabric of Quilt Gate with about 150.
I have never used bed sheets for quilt backings, therefore I can't give you information about this - but I am sure you will get help here on the board.