Limit Homepage Post Native Gallery Thumbnail count

I wonder how to limit the amount of gallery thumbnails shown in homepage recent posts.
in twenty twelve theme when i add a gallery to a post all the gallery thumbnails were display on home page too.

that is pretty weird. please if you can help me sort this out.

If there is a way to conditionally change the thumbnail amount of a gallery when in homepage excerpt and full page that can do the trick, but i’m not that good to figure that myself.

Thanks.