-ingi takes the adjective prefix for the noun class rather than the subject prefix. For N/N (I/Zi, 9/10, whatever you want to call it) adjective prefixes are:
Vowel stem: Ny-
Consonant stem starting D, G, or Z: N-
Consonant stem starting B, V, or P: M-
Other consonant stems for this class take no prefix.
(There are some adjectives that behave strangely in this class but, since none of them is -ingi, we can ignore them for now. They are in the Adjectives tips and notes.)
This class uses the same prefixes for adjectives in both the singular and plural. The -ingi stem starts with a vowel, so nyingi is both the singular and plural form. Given the word brings it's own context it's easy to work out which way it's being used.