If comments are enabled on my site how can I moderate them?
There is a site wide report that admins can access from Site Administration / Reports / Comments. Unsuitable comments can be seen and deleted from there.
Can I decide how many comments to display in my course/block?
Currently the number of comments displayed is set by the administrator in Site administration / Front page / Front page settings and so it is not possible to set this at course or block level. See MDL-25332.