How do I merge more than 12 pdf files?

You have subscription to an online service, and that limits you to 12 files at a time. With Adobe Acrobat (the desktop application), you would not have this limit. If you need to merge 23 files, you can first merge files 1-12 to a new file, and then in a second step that new file and files 13-23. This way, you are never merging more than 12 files, but the end result will still be a merged file consisting of more than 12 individual files.