When you're judging the quality of Egyptian cotton sheets (or any type of sheet, for that matter), thread count is pretty important. But, it's not important in the way that you might think.
We've been taught by marketing and media that higher thread counts mean better sheets, but this isn't the truth at all. Those 1800 thread count sheets that you've been eyeing on Amazon aren't what they seem to be at all and at a $24.99 price point, they're guaranteed to be just a step above sleeping on paper towels. You'll find better than that listed on this site: http://luxuryofthepharaohs.com/best-egyptian-cotton-brands/.
Thread count refers to how many threads there are vertically and horizontally per square inch of fabric. When a thread count is below 180, you'll be able to hold it up to the light and see holes. When you see sheets that are very high thread counts (over 1000), you're dealing with manufacturers that are using multi-ply threads to inflate the thread count to make it sound better than it is.
You're looking for Egyptian cotton sheets with a thread count of between 200 and 600. Much higher than that and you're certainly dealing with lesser quality sheets. The higher the thread count from there, the worse you're getting ripped off.