That is, how many Arabs from British Mandatory Palestine?
Assuming that this figure of 12,000 Arab volunteers for all of WWII is correct (British records from April 1943 showed that a mere 9041 Arabs had volunteered over the entire War period till then), how many were actually involved in combat? British records show that more than half the Arabs who enlisted ended up deserting. And a very small percentage actually made it out of boot camp, and most of those who did ended up on guard duty. Only a tiny portion actually ended up in the theater of war.
British records also show that the effort to create a mixed Jewish-Arab Palestine regiment based on equal conscription from each community was a monumental failure, since the ratio of Jewish to Arab enlistment was about 3 to 1, despite the pre-War Jewish population in Palestine being less than half that of the Arabs.
Roughly 100,000 Jews signed up to join within the first year of the War, and roughly 30,000 were conscripted and served during the War years. Most Jews served in all-Jewish battalions from 1942 onwards, and in the last few months of the War a distinct Jewish Brigade (fighting under the Zionist flag) was formed as part of the British Armed Forces, and saw action on the Italian front.