When one notifies their CLEO of having built an NFA item on a Form 1, is it poor form to include multiple items in a single letter, or should one send multiple letters?

Just submitted a bunch of forms and have never had more than one Form 1 in progress at a time.

It doesn’t matter. Most CLEOs just file them (usually in the round file). The only requirement is that you notify them, period. There is no requirement for the CLEO to actually do anything with them, or any requirement for you to keep a record that they got it. Save a few stamps and send them together if you have multiple ones ready to send at the same time. They won’t care.