What comes after "octuplet"?

Solution 1:

We don't often need it, but the word is nonuplet.

Next after that comes decuplets, who according to the Guinness Book of World Records were born at Bacacay, Brazil on April 22, 1946. (I must admit I'm somewhat sceptical).

It's hard to be dogmatic about the spelling of a word that probably doesn't make it into many (if any) 'authoritative' dictionaries, but I will just say that - Wikipedia notwithstanding - I don't have much time for decaplets. Here's an NGram showing that not many others do either (in fact, there are 5 instances of decaplet in the NGram corpus, as opposed to 2450 decuplets).

Solution 2:

Nonuplets. There doesn't seem to be any record of decuplets, undecuplets, dodecuplets, etc.