This will depend upon what your goals are. If you’d like to simply get an overview of what your basic postural and movement habits are (and how some of them might be creating problems with pain and/or dysfunction for you), then a single lesson is a good place to start. A single lesson is also a good place to start if you’re simply curious about the work and how it might help you.
If you’re hoping to go after some of the deeply ingrained postural and movement habits (those habits that could be contributing to any pain or dysfunction you might have), then I suggest taking lessons in blocks of five until you are confident that you are able to effectively address those habits on your own. These lessons really need to be taken at least weekly, if not twice weekly at the beginning, as movement and postural habits can be tenacious.
And please keep in mind, the Alexander Technique is not something you “master” after a given amount of lessons. The work is not about perfecting anything. Rather, it’s about exploring and improving upon what you already have, based upon your human design. It is a skill you can apply to cultivate continuous growth and improvement for yourself in the activities of daily living, finding ways to move with less pain, fewer limitations, and with more freedom, choice, comfort, safety and satisfaction.