This excerpt come from Landmarks of English Literature by H. J. Nicoll:
It is not to the credit of England that the only full survey of its literature possessing any high merit from a purely literary point of view should be the work of a Frenchman. We have among us not a few writers, any one of whom, if they would abandon for a few years the practice, now unhappily too prevalent, of writing merely Review articles and brief monographs, could produce a work on the subject worthy of so great a theme.
I am not sure why "not a few" is used, because it could mean "a lot of" or the exact opposite "not a single". Does "not a few" always mean "a lot of"? Can you provide some examples and explanation?