Firstly, there is NO limit to the number of images you wish to add to your Gallery!
However, you would need to ensure that the Gallery settings has been updated to display all possible results.
Navigate to your gallery and hit Display Options in the Media field:
Make sure that the Number of items to show is being set to 'Show All'. This allows you to add as many images as you'd like in your gallery.
Check THIS ARTICLE on adding, replacing or removing images from a gallery section.