It varies by school so you should check the application instructions for each school you're applying to. Generally schools will want at least 2 recommendations, I've also seen some ask for 3. Typically there is a strong preference that one come from a current supervisor, although plenty of people apply without their supervisor's knowledge so schools are pretty used to people asking former supervisors as well. In some cases they'll ask you to explain why you don't have a recommendation from a current supervisor. Again - this would all be explained in the application instructions for your school.
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