Five reviews, and no way round it
The Places API returns a maximum of five reviews per place, chosen by Google, with no pagination. It is a product decision, not a quota.
The other API
The Business Profile API returns everything, but only for locations you own and have verified, and behind an access request.
What surrounds the call
Caching, quota handling, rate limits, attribution requirements and display rules. The API call is the easy part.
When to build
Inside a product, across locations you own, or combined with other data. Not to put reviews on a website.
Read the full version
The detail is in the Google Reviews API guide and the five-review limit.