You should see why the notifications are being sent and which rule is responsible for that inside the notification-log in
~/var/log/notify.log. For example:
2021-08-02 18:47:38,793  [cmk.base.notify] Got raw notification (....;....) context with 52 variables
2021-08-02 18:47:38,796  [cmk.base.notify] Global rule '<description>'...
2021-08-02 18:47:38,796  [cmk.base.notify] -> matches!
2021-08-02 18:47:38,796  [cmk.base.notify] 1 rules matched....
Have a look at the log, this should give you a hint.
You should also check if all the users that should receive emails have valid addresses configured in their user-profile. If the user does not have an email-address configured, no mail will be sent for that user - at least that is how it is with your notification-rule configured like that. Maybe currently only 3 users have mail-addresses configured in checkmk, so only these users receive the notifications.
You can also try to add explicit email-addresses (completely decopuled from the users) in the notification-rule and check if the mails are being sent to those addresses.