Stid pretty much nailed it, the migration varies every year. I flew out with BRA to the Howard Pass area a number of years ago. We hunted the first week of Sept. I had a buddy who hunted the week before us about 10-15 mile from where we were dropped. His group saw 200 or so caribou during their week. Our first day of hunting we saw around 50 bou, the next day 100, and the numbers just kept steadily increasing. We figured we saw 1,000-1,200 at varying distance during the five days we hunted. A couple days after we left the field the temperature rose a little and from the reports the migration absolutely stopped moving south.
I have no doubt you'll see caribou during your trip. The question is going to be how many, 100 or 1,000? Either way I should be an awesome trip. We saw a couple of bears and a wolverine as well. The dang wind was terrible each time the gizzly would pop up. Pretty much busted before we could even think about a stalk. Bettles is a few bucks more than flying from Kotz. but much more enjoyable of an experience.
Jay and Judy are great to work with. They have a kitchen with some big counters in the back of the hangar. We spent the night deboning our meat and boxing it up before we flew back to Fairbanks the following morning. They had a couple big chest freezers to get a good chill on everything as well.
I'm still kicking myself for the terrible approach we made on an absolute giant caribou. He still haunts me from time to time.