Every plan includes a monthly quota of review requests (SMS + email combined). The counter resets on your billing date, not the calendar month, and you can see current usage any time from Settings → Billing.
Requests that fail to deliver — invalid numbers, bounced emails — are not counted against your quota. Only requests that successfully send are billed.
If you're approaching your limit, Sadakom emails a warning at 80% and 100% usage. You can upgrade mid-cycle and the new limit applies immediately, pro-rated for the rest of the billing period.