I think there are more than 100 singles in Quad. Almost all of the third floors are occupied by singles exclusively. There are 19 stacks (I think) and each has 5+ singles. So… math = ~ 100.
How quickly they’ll go? You can probably get one. I wouldn’t exactly call the Quad singles “popular”… Some people really prefer them, but a lot of older students enjoy apartment-style living and/or being off campus. I recall many singles being available when I wanted one in 2010-2011 and I didn’t have a great number.