There are no limits we are aware of? run a quick test here with in excess of 1300 fields and all is Ok.
Having said that, the recent update (v1.12) has included some export improvements
Publish your form again (as a new version) and then try to export again to see how your data is there as well (some of these changes will require ‘new version’)
another option though, especially where you are working with large data sets is to use the Connection Agent where you can write the data out to an external database as the form data is submitted by the user