The number of people who have chosen to follow your organization.
Following is something a person does from your organization page. It is not the same as being in your database, and it does not approve them for anything. A follower may never have applied to you, and somebody you work with every week may not be a follower.
No, and this is the part worth knowing.
Notifying followers is a choice you make on each opportunity, using the Notify followers setting. When it is enabled, existing followers receive a notification about that opportunity. When it is not, publishing is silent as far as followers are concerned.

Only followers who meet the semi-private conditions you set are notified. The rest will not see or hear about the opportunity at all, which is the point of semi-private visibility.
Yes. Follower is one of the standard fields on a profile, so you can use it in your database views and reports alongside anything else you track.