how do i download 15 photo folders to my ipad. it only lets me download one. if i try download another folder, it deletes the previous folder.

You need to select and sync them in one go. Photos that you sync from your computer can't be deleted directly in the Photos app, instead they are deleted by not including them in the next photo sync - so you've effectively been telling iTunes to replace the folder that you've just synced with a new folder

A Little further explanation.... You need to place all of the folders into one main photos folder and then select that main photos folder as the Sync Photos From folder. Then you can select the 15 sub folders within that main folder to sync.