I would say roughly 10-14. Most breeders recommend about 10 feeder crickets for a 4-5 month old veiled and those locust, I would say, are just a little smaller than a normal cricket.
If you feed him 10 or so and he doesn't seem to want any more then don't try to force the others on him. Some chams can over eat as well, but I would say at least 10 is a good number.