The maximum number of references that can be exported from an EBSCO database at one time is 5,000. If there are more than 5,000 references in your results that you want to export, use the date slider on the left of the results page to reduce your references into smaller chunks.
For example: if the publication dates of your results span from 1978 to 2012, and a roughly equal number of references were published in each year, you might want to limit your results to all those published 1978-1987 and then export those results, then do the same for 1988-1995, 1996-2003, and 2004-2012.
Please be aware that downloading large numbers of references at once can take a long time.