Answer
The number of sent and not-yet-confirmed contact requests is limited to 100. If you reach this limit you won't be able to send further requests.
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So what options do you have once you’ve reached your limit? If XING members don’t confirm your contact request you should delete them after a certain period so that you can contact other members. You’ll find the list of your pending contact requests here.
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Why this limit? We want to avoid the site being misused, for instance to contact XING members indiscriminately and pester them with anonymous mailings, etc.