It's probably impossible to work it out - if you had a copy of the Folk Directory from that period, you may get some idea. However, that tome always understated the position, as many couldn't be bothered to send in their details (nothing much changes!!).
Bear in mind that there was usually at least one in each small town or village, and the bigger towns and cities had many. In Coventry, where I live, there were over 20 weekly clubs during that period, ranging from small singers clubs right up to the big concert clubs. This was a pattern repeated all over the country - ask any of the performers who were doing the circuit then.
Also, just about every college and university had a folk club back then.
And, of course, in the late 60s and early 70s, they were all pretty full - there were loads of weekly clubs that regularly pulled in crowds of 100 - 300 people. Some more. The total must have run into hundreds of thousands.
However, I suspect that the total number then was not much different to the total number that now go to folk festivals.
The 'open mic' phenomenon is interesting. It is very similar to the folk club movement back then, and in common with it, seems to have grown up very rapidly, and without any sort of central organisation, but by a form of 'osmosis'. Just like the folk clubs then, there now seems to be an open mic just about everywhere, and several in the large towns. The only difference is that the word 'folk' is not mentioned anywhere, and there is very little folk music being performed in them as far as I can see, although there is a sort of contemporary, singer/songwriter, acousticy sort of thing going on (which IS similar to the folk clubs as they developed in the mid 70s.
If there is a statistician on this forum with time on their hands, I'm sure it would be possible to put a figure to it!