Narapatijayacarya 2.2.18

2.2.18

ṣouṃśākharo'vargaḥ syātkādivarṇāstu pañcakam |
caturvagau vargo saṃkhyā varṣeṣu kīrttitāḥ ||

The a-class has sixteen phonemes, while the phonemes of the ka-class and the rest form groups of five;
The ya- and śa-classes have four phonemes each: [these] numbers are declared for the classes.