All block types that are possible to use on the site are displayed. You can change their order by dragging and dropping them. The order will be changed for everyone on the site.
By changing the order you can highlight often used block types or encourage people to add lesser used ones as they are higher up.
Per default, up to 4 block types are displayed when adding the placeholder block to a page. These are the most important ones. The other blocks are made visible by clicking the "Show more" button in the block configuration.
The numbers in each block type on this page indicate how many times a particular block type is already in use. Note that some block types are placed on every dashboard, profile, or group homepage. These numbers may then be higher due to being on everyone's page or every group homepage.