There is no universal threshold, and any number quoted without a city and a category attached is guesswork. The honest answer is relative: you need to be in range of the businesses currently holding the top three map pack positions for the searches you care about.
How to find your actual number
Search your main money keyword with your city attached, look at the three businesses in the map pack, and note their review counts. That range is your target. It costs nothing and it is the only benchmark that describes your competition rather than an industry average.
The spread across Phoenix metro is wide enough that averages are useless. Competitive East Valley trades like HVAC and plumbing routinely need three-digit review counts to compete for top-three positions, while a lower-density category in a smaller West Valley market can be competitive at a fraction of that. Same state, same year, completely different targets.
Volume is only half of it
Three things matter alongside the raw count:
Velocity. A profile gaining reviews steadily outperforms one that gained the same number two years ago and went quiet. Recency is a signal on its own.
Distribution. Reviews that mention specific services and neighborhoods build relevance for those terms. A review saying “great work” contributes to volume; one saying “replaced our AC in Power Ranch in July” contributes to volume and relevance.
Rating. Between roughly 4.5 and 4.8 is the practical band. Below that you lose conversions regardless of count; a perfect 5.0 on low volume reads as new or curated rather than trustworthy.
The practical target
Set a monthly rate rather than a total. Most local service businesses can sustain five to fifteen new reviews a month with a request workflow that fires after every job, and that rate closes almost any realistic gap within a year while keeping velocity healthy the whole time.
The map pack ranking factors cover where reviews sit relative to the other signals, and how to get more Google reviews covers the workflow itself.