Please ask this question directly to the Limesurvey GmbH via the "Contact Us" link. In the forum, most of us do not have this kind of information. But, if I recall right, you just need to buy another monthly package and you get the respective amount of responses.
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
No support via private message.
