Select as many files as you like at once on the upload page of the event, or drop them onto it; they are uploaded one after the other and every file shows how far it is. The page names the largest file your server accepts and does not send a larger one; the limit is the smaller one of the PHP settings upload_max_filesize and post_max_size, so ask your host if it is too low. You can also use (s)FTP to upload your files to /images/eventgallery/[yourfolder] and use the Sync Database button to make the new files visible to Event Gallery.