Complex sentence: In that time 133 NCB suffered 328 casualties, including 3 officers and 39 enlisted killed in action plus 2 MIA, which was the highest number of casualties for any Seabee unit ever.
Split into simpler sentences: In that time 133 NCB suffered 328 casualties, plus 3 officers and 39 enlisted were killed in action and 2 were MIA. These losses were the highest number of casualties for any Seabee unit ever.