The Amplify tab will give you access to this figure.
Under the 'Step 1' header, you'll be shown how many recipients you have left and when your current subscription will renew. Your allowance will reset upon your renewal date, but if you use up your allowance before then you can easily top up.
To top up, you can either contact your Account Manager, the Support Team or click the 'Get More' button. This will provide three different email bundles. The instructions are shown in the screenshot below.
After submitting, you will then receive an email from the Support team confirming your selection before it is processed.