This is a little more modern than my area of expertise, but if I recall my education, L. Yablonsky estimated between 200,000 and 400,000 hippies during the 1960s.

http://www.worldcat.org/title/american-hippies/oclc/900332629

http://www.worldcat.org/title/hippie-trip/oclc/647315095?referer=di&ht=edition

Perhaps looking at music and literature charts from the era might provide us with some information on the popularity of certain societal changes, movements, trends and fads. Has any historian here examined those sources and can provide us with some insight?

One of the quintessential disco albums was the Saturday Night Fever soundtrack. It epitomized the entire era, a collection of some of the most popular hits of the 70s that were in the Saturday Night Fever movie with John Travolta. It's sold 15 million records in the US, topped the charts for months on release and stayed on the charts for over 120 weeks. There's several other things you can look at to see the popularity of disco at the time, but that is somewhat indicative.

This question is really hard to answer without clarifying what you mean by participate. You can be aware of a movement and even sympathize with it without joining the more extreme forms. Relatively few went to a "love-in", but a great number could hum a few bars of "Love the One You're With". Roughly a quarter of adults have gone mixed gender skinny dipping while only very few went streaking. Buying a button down shirt in the late 70s it was hard to avoid the disco fashion trends.

As others have mentioned, you'll need definitions beyond mere consumer preferences ("I owned a tie dye shirt back in the day, so....") to meaningfully distinguish participants from everyone else, which seems to make any collection of "trends" truly apples and oranges. For instance, to me, being a part of the hippie era would seem most definable around political participation; however, the disco era/"me generation" seemed defined in part by pulling back from formal politics.

With this approach in mind, for the hippie era I'd start with voting patterns, something where there's good concrete data. For instance, in the U.S., how many people voted for Eugene McCarthy for President in 1968? (an overt protest candidate) Likewise, how many voted for George McGovern, a liberal Democrat in 1972? (Although McGovern did win the party's nomination, and hence appears more "mainstream," Richard Nixon had a famous quote to the effect of "knowing" he would win reelection as soon as he learned of McGovern's nomination, which suggests Nixon saw a clear distinction between McGovern's supporters and everyone else.) In both cases the number of supporters were quite small, and even if you add in more fringe candidates on the Left, by this definition the hippies were far smaller than popular culture/consumerism makes them appear.

Fwiw, these sorts of questions are really worth asking, even if they're often more challenging to answer.