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or do you need to use more than 5-7 lines?
My question about this is: "need to" for what purpose? It depends very much on what you're trying to achieve in your practice and with the end product of your drawing. However, for 30 second gestures, the usual "point" is to help warm up your brain and get it ready to see and make decisions about what is most essential to a pose, what will convery the feeling or energy of what you're seeing as quickly as possible. That's why the length of time is so very short - you're not really supposed to be able to capture an entire figure in that length of time, you're supposed to have to make hard decisions about what to capture and what to leave out. The idea of limiting how many lines you use is a great one for this same sort of training. For most people, the short length of time just naturally limits the number of lines they can make anyway. For others, adding this second layer of restriction helps to sharpen their thoughts and learning process even further.
Of course, if you are making a finished piece of art that is meant for display, it's entirely up to the artist how many lines they want to include. But when it comes to exercises that are meant to teach you something, push you out of your comfort zone, and don't necessarily need to be finished pieces you hang on a wall -- I think this can be a useful exercise for a lot of people. :) Depends on what you're working on learning right now though!