1. Question: Why do we differentiate blessings between one vessel and more vessels when dipping them in the mikvah but when lighting Chanukah candles we say the same blessing whether for one candle or more?
A. The Pri Megadim says we say “lihadlik ner” in singular form for all nights of Chanukah so that we don’t differentiate between the first day and other days of Chanukah.
B. The Mishna Berura (263:22) quotes the Pri Megadim to say that for Shabbos candles we say “lihadlik ner shel Shabbos” no matter how many candles you light because the main obligation is one candle. Technically you only have to light one candle a night on Chanukah too.
C. For dipping vessels into the mikvah you say “al tevilas kli” for one and “al tevilas keilim” for more than one. (Dirshu Mishna Berura 676:1:1:1)
Answer: By tevilas keilim each vessel is a separate obligation so one should say it in plural for to bless Hashem for each mitzvah you do but by Chanukah each added candle is only beautifying the mitzvah, though that becomes part of the mitzvah but since technically only one candle is needed per night then we stick to singular form for the blessing.