Quoting @jeherve from #10132 (review):

The block inspector offers me the option to display up to 3 posts per block. It would be nice to have the option to display more, especially since that is possible with the existing Related Posts (through a filter).

Via Automattic/wp-calypso#30101:

by default, the maximum number to be displayed is 3. Soon will the max be at 6 ?

It would be nice if the block allowed you to insert more than 3 posts, just like you can customize the number of posts returned with the regular related posts:
https://jetpack.com/support/related-posts/customize-related-posts/#number

Requested here:
https://jetpack.com/2019/01/10/jetpack-6-9-introducing-more-tools-for-the-new-block-editor/#comment-193506