how many users typically use the "forgotten password" flow?

Solution 1:

Anecdote alert

From personal experience it's the sites that I don't access very often where I forget my password and have to access the "forgot my password" flow. For sites I access frequently I can remember them.

So make sure that the pages are very friendly for the occasional user.

So from my small sample of 1 the usage rate would be inversely related to the visitation rate. Frequent users won't use the option very often where occasional users will.

I know this doesn't directly answer your question but might give you some insight into the type of user who'll use this option.