You could make a book, add all 21 individual ID docs at once, then export from book file to make one PDF with 21 pages.
I'm not sure I knwo how to make a book. That is different than
printing a booklet, right? I'll search in help sreeens but "humans" somehow make toi more understandable.
To create a book, File > New > Book (third one down). Once book is created, select the flyout from panel and select Add Document... Browse to location of your letters and select.
You might have been able to select them all at once, right click therm and choose convert to Acrobat PDF. Other wise google for Peter Kahrel batch convert and download that script.
Any adding of form fields has to be done in Acrobat though, your "memory" of doing that in InDesign or upon export is impossible.