For comparison: Regular quilting cotton averages 68-75 square, pima cotton (hand dyes usually and some batiks) is about 133 x 72, batiks are in the 200-300 range.
Remember that some sheet manufacturers count the fibers in the threads, not the threads. So, if there are three fibers twisted to make each thread, they call it a 600 thread count when it's really 200. It's not really apples and apples to compare thread count in sheets to commercial fabrics unless you consider that aspect of it.