You have three different systems you can choose from:

Free: You can have until 500 subscribers, send until 2500 emails per month, and you don’t have the autoresponder option.
With limitation of subscribers: You can have subscribers within a certain limit, and send unlimited emails. Only active subscriber count (if a subscriber leaves the list or if the email address is invalid, it doesn’t count) and if a subscriber exists in several lists, he counts in all of them.
Purchasing credits: There is no subscriber limit and you purchase credits as you need to send emails.

1 credit = 1 sending, 1 sending is a sent email, in a campaign or in the autoresponder.
Choose the best option for you. At any time you can change the level or the system. When you change level or system, your balance is automatically converted to the level/system you choose.