[noun] + は/も/が + 少なくない [verb (plain)] + ことも + 少なくない
少なくない is the negative of すくない少ない, "few", and means "quite a few", "not uncommon". The thing being counted takes は or も, often after a clause that describes it: "people who think so are not few".
It belongs to writing, news and formal speech. In conversation you would say おおい多い or けっこういる.
These go beyond counting people or things.
After a clause and ことも, it says something happens fairly often.
少なくない is an い-adjective form. Its polite forms are すくなく少なくありません and すくなくないです少なくないです, and its past is すくなくなかった少なくなかった.