Spark: I think you probably dismiss a square if it contains other squares or rectangles. Undoubtedly there are 11 squares although I must share your concern because for some reason I count only 10

...while for obvious reasons I find it difficult to specify the one I can't find

Edited to admit sheepishly that I had missed the middle 2 x 2

.....the one Zed has described as "that one there"