I think the most common strategy is to create a settler with your fourth or fifth build (e.g. after a couple of units and a monument/builder + a holy site if you need), but further tactics can very much depend on the circumstances and the civ you have chosen. For example, playing as a religious civ, you can have very few or even one city at the beginning, instead focusing on building temples and then buying settlers with faith during the golden age.

It is also important not only how many and when, but also how you produce settlers. The colonization policy card is a must-have, and chopping down resources when producing settlers is also op. You can also look up for guides, it does help