1)
What is the significance of the number of Kohanim?
1.
Malbim: Two Kohanim must blow trumpets. It seems that they also sang. There were 12, just like we do not have less than 12 [Leviyim] on the Duchan. Since it was only a commemoration, there were 12 in all 1 .
1
The previous verse mentioned nine Kohanim, and then, including our verse, it mentions four playing instruments. Three (or all four, if Yachazi'el is Yechi'el) were among the nine! Perhaps Malbim adds two more from "[Asaf] v'Echav" in the coming verse. (PF)
2)
What are Chatzotzeros?
1.
'Rashi': They are a musical instrument of Kohanim, like above (15:24) "u'Vnayahu ve'Eliezer ha'Kohanim Mechatzrim ba'Chatzotzeros."
