At what rank does having a main become less effective?

As a person who recently started playing ranked, i was rather lucky in being placed gold III, having seen video's made by various youtubers they generally agree that having a main is a good way to climb - so i chose nami {{champion:267}} . However my proficient champion pool is quite small and i'm wondering when i'll need to diversify my champions or if I will be fine continuing to predominantly use nami {{champion:267}} and zilean {{champion:26}} ? Thanks for the help!
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