I can’t answer your first question about the number of revolutions, however I can answer the broader topic of the subversiveness of Salon/coffeehouse culture. The short answer is depending on the coffeehouse it could be either. There were coffeehouses that held revolutionaries and coffeehouses that housed reactionaries.

To explain how such cultures could be as such we need to look at what coffeehouses were. What they were not, is Starbucks. If I were to give a modern example of a coffeehouse it would actually be social media. Coffeehouses were places where people gathered to discuss ideas over coffee and tea rather than the coffee and tea being the main attractions. They were where one went to get the news from the colonies and discuss politics, technology, and all sorts of topics. Thus they often were seen as subversive because the rulers focused on the coffeehouses that spread subversive thought and not the ones who supported the current regime.

For instance, in James Van Horn Melton’s book, The Rise of the Public in Enlightenment Europe, which discusses Coffeehouse culture and other spheres of the rise of the Public in Enlightenment Europe, an example of how coffeehouses could be both subversive and loyalist Charles II of Englands advisors debated about the subject, “when the earl of Clarendon urged Charles II to suppress coffeehouses, Sir William Coventry reminded him that in Cromwell's time "the King's friends had then used more liberty of speech in these places then they durst in any other."

So we can see that in this instance, Coffeehouses were as much a boon to power as they were a creator of revolution. There are more reasons than just paranoia about the subversive thought in coffeehouses that fueled the popular idea of it.

Before coffeehouses, the most popular places to meet and discuss were taverns where they sold alcohol. Alcohol is a depressant, while tea and coffee are both accelerants. People took note of this at the time and made correlations of how taverns generated more relaxed speech and not subversive speech. Meanwhile the accelerant effects of coffee and tea were both seen as creating wild, subversive thought and speech.

So in the end, the fears of subversive thought and revolution in coffeehouses was founded in truth in that there were many revolutions planned in coffeehouses, but coffeehouses were not solely bastions of subversive thought and some held reactionary and loyalist thought as well.

For a more more reading I recommend The Rise of the Public in Enlightenment Europe by James Van Horn Melton. Dorinda Outram’s The Enlightenment (New Approaches to European History) also had a chapter about coffeehouse culture.

Edit: had to edit out the title I put in my notes app to write this comment and accidentally pasted in.