Yes, the number of sent and not-yet-confirmed contact requests is limited to 100.
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.
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.