HOW TO TRANSFER ALL the pics all together

Are you talking about photos synced to iCloud? If yes, there is no way to easily check how many have disappeared. You would have to know how many you started with and then determine how many are left. If you take this approach, them be sure and look in the "Recently Deleted" section of iCloud Photos. Also, if you have many photos, it is not easy to transfer the all together. In this case, you would need to transfer them in batches. To transfer them, I would use Download iCloud photos and videos  https://support.apple.com/en-us/HT209454.

Also check for a Recently Deleted album in the Photos app.