Just from my own knowledge of my local area (which itself isn't comprehensive) I can see that the Living Traditions lists are far from being complete.

It is probably impossible to know how many clubs and other folk events there might be. Lots of smaller events are still publicised by word of mouth or very local advertising and may not have a web presence. I know of one local folk evening which is publicised through the pub's website and doesn't show up if you google "folk club" I am confident that any attempts to calculate the number of clubs will substantially underestimate the correct figure.