וַיֵּצֵ֞א הַגּוֹרָ֧ל הָרִאשׁ֛וֹן לְאָסָ֖ף לְיוֹסֵ֑ף גְּדַלְיָ֙הוּ֙ הַשֵּׁנִ֔י הֽוּא־וְאֶחָ֥יו וּבָנָ֖יו שְׁנֵ֥ים עָשָֽׂר׃
Now the first lot came forth for Asaph to Joseph; The second to Gedaliah; he and his brethren and sons were twelve;
English Explanation of Mishnah Arakhin
There were never less than twelve levites standing on the platform and their number could be increased into infinity. There always needed to be twelve Levites standing on the platform in the Temple singing. This would mean one for every lyre (nine), one for every harp (two) and one for the cymbals. This number is also referred to in I Chronicles 25:9.