In Shilo, there were only 16 Mishmaros - eight from Elazar and eight from Itamar (Ta'anis 27a). Why did David make 24?
'Rashi' and Malbim, from Ta'anis 27a: He saw that there were many Roshei Giborim from Elazar, so he split each of Elazar's Mishmaros into two. A support is "Echad Achuz l'Elazar Achuz Achuz l'Itamar" (6). Radak - Mishmaros were added to Elazar like they had initially 1 , but Itamar stayed the same.
Refer to 9:22:1:2.
Radak: Some say that Moshe enacted four from Elazar and four from Itamar, Shmuel increased to eight from each, and David added another eight. Some say that Moshe enacted eight from each, and Shmuel and David added another eight - "Yisad David u'Shmuel ha'Ro'eh be'Emunasam" (9:22).
