There is a 20K email address limit on the number of email addresses you can add to your account using the API using our bulk load method, which is mentioned here and described more fully here.
Trial accounts are limited to 100 or fewer contacts in the account. If you do upload more than 100 contacts while in trial, the system will take you out of the trial period and ask you to pay for services the next time you login through the UI.
Also, it is worth noting that the normal method for adding contacts though the API is here, but I wouldn't recommend adding more than 25 email addresses at a time with that method.
I hope that helps!
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