The Sites Monitor List block is a Gutenberg block which contains settings. When you select the block from the wp-admin and look at the block settings, you can insert how many sites to show per page with the “items per page” setting.
Prior to version 1.1.0:
The Sites Monitor List block uses the WordPress “post per page” setting for its pagination. In a brand new WordPress installation this number would be set to 10, if you want to change the number to something higher for example 50 you can change the setting at: Settings > Reading > Blog pages show at most 10.