One Role or Many in Ranked?

Hey guys, So I've heard that in ranked, you should play with ur champion and role that you are comfortable with. I've also heard of Diamond/Challenger players only playing one role/few champs. I agree with this, but I'm concerned that someone takes my role first and my champion could be banned. Which option would you prefer? And if u prefer the option of many roles in ranked, is there an easy way to master the other roles, esp adc and jg? I feel like for each champion, you not only have to master the role but also have to know your matchups. Thanks!