According to the documentation the limit for sending documents to Portal is 1000 files, but in fact the limit is 500 documents (source file and rendtition are counted as two files).
This is unclear, please set the limit to 1000 documents (with or without rendition) or be clear in the documentation when it is not possible to raise the limit to 1000.
Additional it is possible to select more documents than the actual limit, please issue a warning or error when the selection exceeds the maximum number allowed.