Blog Page in Squarespace show 20 posts/page only. To remove this limit, you can use either of two options below.
#Option 1. Use multiple Summary Blocks
You can use Summary Block and connect with Blog Page
Summary Block has a limit 30 items, but you can use multiple summary block with Filter option (seen screenshots below).
When you add a Summary Block, you can choose this option.
and choose the corresponding Blog Page
To add multiple summaries, you can use Filters option
The idea is that you can assign category 1 to the first 30 blog posts, category 2 to the next 30 blog posts, and so on.
#Option 2. Use Plugin
You can use Lazy Load Plugin, when you add plugin, it will add some options to Summary Block
You can click Lazy Settings and enable this option
You can set some options here
