You can find your monthly contact count on the home screen of your account when you login. This number will reflect, for billing purposes, your active contacts + archived contacts as sort of a high water mark. This will update at the beginning of each billing cycle (ex. you archived 2k contacts in August, this would still count towards August's count but would be updated to your active contact base number in September.).
I am also including a screenshot of this section on your home screen.