AFAIK, the native mail app lets you add only five photos at a time. The reason: You send a boatload of photos, you're probably going to clog your mail, as well as that of the poor slob on the receiving end. Some mail providers even set limits on how big an email can be, and photos tend to be big. (Possible exception: Compressed, or zipped, photos can end up smaller, but I don't know if you can zip photos on iPad.)
If you have a lot of photos to share, I suggest you consider Dropbox or one of the many photo-sharing sites.
