In addition to following aSag's advice about seeing what the schools actually want, just pick the ones your intuition tells you are most likely the "best". Within that priority, try and represent yourself as "balanced" as possible. Don't send them 5 science professors. I sent two science professors, my PI, my employer (worked 2.5+ years), and a more personal one from the supervisor of a very significant and long-lasting volunteer thing I did. I'm happy with that because I feel like it covers academics well and also accentuates my work ethic and non-academic character strengths.
Adopt that philosophy, adapt it to your own application and ECs, and it should become pretty clear which ones to send.