just learn 3-5 champions at each role and sometimes champions will overlap roles.
like j4 mid / top / jungle i'd recommend him at
or like velkoz support and mid
annie
zyra
morgana those 4 mages can support or mid just fine
warwick / j4 / lee / rengar those 4 can easily top or jungle just fine
and for adc, me personally i just play caitlyn only, and if i cannot get caitlyn, i try to go ashe real basic adcs. whenever i play rank, i just go fill and generally this is the guidelines of champions that i use myself personally
zyra / vel / blitz / morg / j4 / panth / wukong / caitlyn / khazix now if i had to pick a "main" i'd say imma jungle main, but can play each role just fine with those 9 champs posted.