I am not aware of any limit, but I know it would be larger than 6. There is a limit of uploading a maximum of 20 images at a time. If you click the gallery and then manage gallery there is an add images link at the top. You should be able to upload more images there. Then you can select them for the gallery.
The website is http://www.hunterquarterhorses.com and page "For Sale". You see 6 pics in the gallery. When I try to add 7th & 8th in edit mode (see below pic) the 2 new ones I am trying to add are checked and I hit save. Then when I try to preview - they won't show. I am very confused as to why.
Drew - Thanks for your time! I thought the gallery main page would show ALL 8 pictures vertically (it only shows 6) ; then if you click on any pic it takes you to the "scroll thru" view you have in image above. Which DOES show all 8 pictures to scroll thru. Am I wrong about how main "For Sale" page works?