How can I increase the number of posts my Squarespace site shows per page?

The Squarespace editor limits the number of blog posts per page to 20. I would like to increase this limit, but I don't know how or where to do that.

Is this possible?

Unfortunately, no. You just have workarounds. Summaries have a 30 limit, instead of 20, then again, you can also "chain" them by putting up multiple summary blocks. This is the way most do it.

Then again, there's this: "The lazy load summaries" plugin that has an option to go over the 30 item limit.

There's no "real" native way to do it other than with summary blocks or plugins (and even then, not for the actual posts). For now, anyway.