It's hard coded to 100 but I can't immediately recall why other than there's a fairly high chance that you'll run into potential server limits if it's higher. You'll potentially hit things like post_max_size, max_input_vars limits in PHP, you may exceed max_execution_time or experience issues with memory or CPU.
If this is just temporary, you can try manually increasing it by modifying the following files:
Okay so you can set 100 images to a post but only upload 50. Is this a bug? On previous versions to coiuld edit the pages and set whatever was required based on your server resources.