How do i do to get silver ?

So yea im rn level 21 , but how do i do to get Silver after the placements? I know winning the placements have nothing to do with it, my friends won 10 of them and ended up Bronze 5 lmao. Is it something with the wins and looses in normal games? How many wins do i gotta have to get a good elo in the placements ?
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