Your quota is the number of email validations you may use, as shown in the top right-hand corner of your account, under quota remaining. This is set by your subscription plan. Any unused quota will not roll forward into the next billing cycle.
If you upload a file that contains more email addresses than you have available in your remaining quota. You will need to upgrade your subscription plan or pay for the file to be validated as a one-off payment.
The system won’t let you go over your quota limit and will instead take you automatically to a payment screen, where you can choose how to proceed.