You’re making a very interesting point about critical size. We felt this in out team as well.
As soon as we reached the magic number of 3 devs, we started to get into the habit of peer reviewing ALL our code (to catch bugs, and also learn from each others).
This was virtually impossible to sustain with 2 people only. Sometimes I wanted to merge some code asap but my colleague was busy, so I had to merge without review. With more people, we usually find a reviewer available anytime.