Thread count is the number of lengthwise and crosswise threads in one square inch of fabric. The logic looks simple: the more threads, the denser and better the fabric. But there is a little sleight of hand involved.
Some manufacturers twist two or three fine yarns together and use them as one thread, then count each of them separately. On paper you get 1,000 threads, but the fabric in your hands may be no better than a 400-thread fabric woven from good single-ply yarn. Often it is worse, because those fine yarns are weaker.
Then there is breathability. The denser the fabric, the less air passes through. The result is heavier, warmer and sometimes slightly "plasticky". To give an honest answer, a duvet cover between 250 and 400 threads, woven from good cotton, is the sweet spot for most homes. As the number climbs higher, it pays to look at it with suspicion rather than curiosity.