Most people don't realize that 100 people could fit in their living room, when standing shoulder to shoulder. Arial pics can be evaluated fairly accurately: 10 rows of 10 doesn't take up that much space. "Hundreds" would be a very small crowd.
I was there and stood on the fence in front of the State House. The crowd was enormous, spilling into the Boston Common. I think the 10,000 estimate is close to accurate. In any case, it was much, much larger than the Free Justina Pelletier rally of two months ago, and that crowd numbered in the thousands.