There is no fixed number. Anyone guaranteeing an exact count every month is buying them from the same handful of sites — and it shows in your profile.
When a piece is paid for, we say so openly and mark it as sponsored — Google requires it. The real work, though, is having something to say without paying.
You can, but it costs more. If the category pages are not ready, the links pour into a page that cannot rank.
The first placements usually land by the second month; the effect on rankings accumulates slowly. This is long-term work, not a one-month campaign.