Is there a twelfth word to be added to the eleven of the angelic host? After the individual angel spoke to the shepherds keeping watch in the field, Luke 2: 8-12, the multitude of the heavenly host appeared, praising God, and they spoke. I would suggest that their recorded words are a summation of all that was said at the time.
That summation is precisely eleven words in the recorded Greek, given by Luke in chapter two, verse thirteen:

δοξα εν υψιστοις θεω και επι γης ειρηνη εν ανθρωποις ευδοκια
doxa en upsiotois Theo kai epi ges eirene en anthropoi eudokia

This can be rendered in exactly eleven English words (see Young's Literal Translation):

Glory in highest, Godward; and on earth, peace: among men, goodwill.

One would have expected twelve words (the number of covenant), given that this is the inauguration of the New Testament.
Is there a word which is expected to be added. On our part, to these heavenly words of the angelic host?
The only additional word that could reasonably be expected to be added, on our part, is Amen.

Those glorious sentiments absolutely deserve and receive my personal "Amen".