Klaviyo's billing structure is based on two criteria: number of contacts and number of emails. Every plan will have contact and email limit. For example, if you're on our free plan, you will have a limit of 250 contacts and 500 emails. This means that you cannot have more than 250 contacts in your Klaviyo account, and you cannot send more than 500 emails to these contacts.
How Klaviyo Bills Users