A yard of cement is 36x36x36 = 46,656 cubic inches (ci).
If you pour 4" thick, 192x192x4 = 147,456 ci or 3.16 yards. If you pour 5" thick, 192x192x5 = 184,320 ci or 3.95 yards. If you pour 6" thick, 192x192x6 = 221,184 ci or 4.75 yards.
You don't want to run out when doing a pour. Its better to have a little extra. And a place for the cement truck to dump the extra. They don't want to drive away and let it harden inside the truck. I would plan for ordering 4 yards and make the average pour depth about 4.5 inches.