Oh, I just realized based on a bit in your first message that it sounds like you're not editing the correct option. The option in question is "Maximum Images Per Message". That's in the messages option group (not attachments).
Mine works just fine.
BTW, the reason mine is so large is that we have documents we tend to share not just photos. And some of those documents are a very large size.
There're different between attachment and images in messages. And the error message is clear that only 10 images are allowed, so you can set 20 or more attachments you can't have more then 10 images in the message. After you set the limit of images in messages above 10, you can use more then 10 images.
Mine works just fine.
BTW, the reason mine is so large is that we have documents we tend to share not just photos. And some of those documents are a very large size.