I do not think that the number of areas of future work (let’s call them prospects) is a problem per se. Rather, your prospects should be good, i.e., they should be substantiated by the thesis and by possible additional arguments and not be too speculative. This should give you a natural limit, as a given thesis should can only allow for so many good prospects.
Of course, it can always happen that a careless examinor is put off by the plain number of prospects in your thesis, but that’s a risk you have to estimate yourself. This applies to too many prospects as well as to few. An often quoted rule of thumb is that a good thesis raises more questions than it answers.
If you frequently communicated your research progress to your supervisor, they should already know how productive you were without looking into your thesis. Moreover, if your research was comparably unproductive for some reason, writing what could have been done can mitigate this to some extent. This in particular holds if the unproductiveness is not your fault (e.g., if the particle accelerator you were supposed to be working with was unexpectedly out of order for a year).
As always, you should probably talk to your advisor regarding the specific expectations for your field and type of thesis, the pecularities of particular referees as well as whether individual prospects are good or bad.