You can see the number of uses for an activity plugin under Site administration ▸ Plugins ▸ Activity modules ▸ Manage activities in the Instances column. Clicking on the number lists the courses where it's used, a plugin may be used more than once in a course so instances may be more than the number of courses listed.
Similarly for block plugins this is under Site administration ▸ Plugins ▸ Blocks ▸ Manage blocks.