There's no fixed number - ranking depends on review count relative to local competitors, review recency, and review content (mentions of specific services and locations help), not a universal threshold. That said, in most competitive metros, having noticeably fewer reviews than the top three map-pack results is a real disadvantage.
The more useful benchmark than "how many total" is "how many in the last 90 days" - a steady trickle of five to ten recent reviews often outperforms a static profile with fifty reviews that stopped coming in two years ago.