In Mussaf of Rosh Chodesh, from MarCheshvan to Adar II, we add the phrase "ulChaparas Pasha". The Rav has already addressed the reasoning for this. My question is: Do we add an Amein to the congregation’s responses? That is, do we keep the existing Amein after "v’lis;lichas avon", and add (a "new" one) after the added phrase? Or do we keep the same number of Ameins, by combining (Amein-wise) the added phrase with the prior? Thank you.

Shalom, Thank you for your question. As far as I can ascertain there is no set custom. In fact the prayer itself ends with a “let us say amen” at the very end of it – which seems to indicate that originally amen was only said at the end of it, and not in the middle, as we do today. So, as far as a “ruling” on your question goes, it would seem that one should just follow local practice, and either way is acceptable. However, if there is no local practice to the opposite, it would make more sense to add an extra amen after the extra phrase, so as to stress the extra words in line with the extra month. Blessings.