When you upgrade to a monthly billing plan of Mr. E tool, your billing cycle is starts on the day of your subscription start date.
If you started your subscription on 1st then every 1st of the month, your subscription will automatically renew until cancelled.
So on 1st of every month your new credits will be added to your account and any credits which are left in previous month, will also carry forward to the new month.
If you have 100 credits in previous month and on 1st you got 100 more, then this month you will have 200 credits.
Note - If you cancel your subscription in any month, then your credits will not carry forward further and they will expire by the end of the subscription cycle.
To view your account's credits usage, simply open your Mr. E dashboard.