There are so many levels of "TOP SECRET" (channels, SAPs and other restrictive markings). TOP SECRET is essentially the beginning of "actually important" (and I believe there is even plenty of SECRET stuff with restrictive markings which is thus more sensitive in practice than generic TS; some of the restricted markings are in US Code explicitly and pre-date the C/S/TS system). The "SI" part of TOP SECRET/SI was probably more significant. The presentation itself wasn't hugely sensitive (somewhat informed speculation -- at least based on the parts which were published, and a generally informed but not specifically informed review). No specific collection techniques, etc.
I think the published numbers are 4.5mm clearance holders (mostly S or the DOE L; I've personally never met a C-holder); 900k TS+ (including TS/SSBI, etc.)
Essentially every military officer is S or better, I think, and virtually everyone in certain military units is S (IT, etc.) or in some cases TS (CI). That is a lot of people. A lot of 18 year olds :(