If I tovel (immerse in a ritual bath) a glass or metal pot, I recite a blessing first, as for many a mitzvah. The blessing ends "vetzivanu al tevilat keli", "commanded us about toveling a VESSEL". If I tovel multiple glass or metal pots, the blessing ends "vetzivanu al tevilat kelim", "commanded us about toveling VESSELS". However, if I tovel a porcelain vessel, I don't recite any blessing at all, since the porcelain dish only possibly requires toveling. Now suppose I tovel a glass pot and a porcelain dish. Do I say "vetzivanu al tevilat kelim" or "…keli"?
The Aruch Hashulchan in Yo'd siman 120 siff 22 brings from the 'gedolei achronim' that there is no real requirement to make the bracha in singular tense. He adds that the minhag was in fact to also say al tvilas keilim even on one keli.
Whether or not you choose to follow this practice, I have seen that at least in a case such as yours, his opinion is good enough to rely upon and say al tvilas keilim.