Bonjour Hamed,
Usually it is recommended to have 5-10 GCPs per sub-projects. However it is possible to merge sub-projects that do not contain GCPs.
In the shared area it is good to have GCPs, but it will be more important to add MTPs (manual tie point) that are necessary for the merging.
I would like to mention that you are very optimistic with your project. Because having 56 flights with an average number of images about 500 images would result in a 28000 images projects which is really huge.
Largest Number of Images
If you do not have a very powerful computer, I doubt that you can really merge all your sub-projects together.
For such large project, it is recommended to work with large frame cameras.
Hope that you will make it with you project.
Best,