несколько - several, a few. It's 'some' in the sense of 'unspecified quantity', usually more than 2 and less than 10. Usually less than 8 as well.
Sometimes I have an argument about 'can 3 be counted as 'несколько', or 'is 11 несколько' etc.
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некоторый - usually in plural. некоторые, некоторых, некоторым...
It's 'some' as in 'some people say:...'.
I.e. 'subset of countable objests', part of a some* set of people/object but not necessaly all of them.
'some of' - 'некоторые из +some of them - некоторые из них.
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Don't you niticed the asterisk * above, in 'some * set'??
This is the use of English 'some' that is properly translated with Russian некоторый in singular!
некоторый Х in singular means that we aren't going to specify which X.