We assume the best sheets have the highest thread count. We've been duped

"There's a maximum number of threads that can fit into a square inch of fabric," explained Scott Tannen, CEO of Boll & Branch, a luxury linen provider. "Depending on the type of cotton used, that number is generally not more than 400. So there is an awful lot of interesting math involved in the sheets you see in a department store that can be up to a 1,200 thread count."

What is thread count?

Thread count is the total number of threads per square inch in a fabric (counting both horizontal and vertical threads). In theory, the higher the single thread count, the softer and higher-quality the sheets. Unfortunately companies are now splitting the single ply thread and creating 2 and 3 ply cotton threads to market a bed sheet as a higher thread count. Splitting the thread decreases the strength and longevitiy of the cotton thread, it also increase the thickness of the thread which means that there is less space between the threads and therefore the sheets do not allow air flow.

Does thread count matter?

Thread count is, at best, an unreliable metric for buying sheets. Brands nowadays are counting multi-ply threads, which can lead to higher, erroneous numbers.

"In reality, to achieve a higher thread count manufacturers are generally using a lower grade of cotton that becomes very thin when spun," Tannen explained to us. "They then twist this thread around itself to create a `multi-ply' thread. When they use 2-ply thread and weave it to a theoretical 300-thread count (150 horizontal, 150 vertical) they call it a 600 thread count sheet and sell it that way."

So imagine that a 4-ply thread is woven as a 200 thread count, but sold as an 800 thread count. A regular ply 300 thread count would feel better and last longer, but most consumers are convinced to always buy a higher thread count.

 (extract from business insider online - https://www.businessinsider.com/guides/home/best-thread-count-for-sheets)