I think it'll still be cumbersome on your iPhone, but technically achieves what you are asking..
Rather than share a file from the files app 18 times, start with a new email in the mail app, then use the file attachment option to go and select your files 1 by 1. I did not see a way to select multiple, but they would at least be in one email like you want..
If you have a Mac, would become easier as you can select multiple that way.
I don't have a windows PC with iCloud on it to test that way, but I suspect you could do the same thing.
Add email attachments in Mail on iPhone - Apple Support (TM)