Complex sentence: Of 6th Mechanized Corps' 1212 tanks, only about 200 reached their assembly areas due to air attacks and mechanical breakdowns, and even they ran out of fuel by the end of the day.
Split into simpler sentences: Of 6th Mechanized Corps' 1212 tanks, most never reached their assembly areas due to air attacks and mechanical breakdowns. The about 200 tanks that did, ran out of fuel by the end of the day.