EndNote uses Word's hidden field codes when formatting citations, and inserts all the reference information for all of the grouped citations in these codes. However, there is a limit to the amount of text that Microsoft allows in a single set of field codes. You may reach the maximum limit when you attempt to insert 20 or more citations for references that have extensive information in them (such as large abstracts).
To avoid this issue, try not to insert a large group of citations all at once. Instead, insert a small group of references, then a space, and then insert another small group of references. This avoids having a single group of formatted citations that would exceed Word's field code limitations.
When you use Insert Selected Citations, or highlight multiple citations to insert from the Find Citations dialog, EndNote limits you to 250 citations in a single group.