Rather than looking at overall numbers, your best bet is to have a safari planner check on availability at actual camps or lodges. The prices are what they are for peak season, although you might find a "4 for 3" deal or something similar.

You can get an idea by checking the availability calendars of some of the camps -

http://kicheche.resrequest.com/reservation.php?20+++++RS4+.shtm

http://chelipeacock.us.resrequest.com/reservation.php?20+++++RA1+

Thanks for the info. It seems that there is is little difference in availability between July, August, Sept and October.

I'm not worried about finding a place to stay, but I'd just rather go when there are fewer visitors.

'fewer visitors' is relative.
Good guides will keep you away from crowds and you find your own sightings. We often see lots of vehicles watching a sleeping lion and laugh, but head off on our own and are often surprised with cheetah, leopards, other and no one is around.

Of course, if visiting for the 'migration' you have to expect so too are others, so the banks of the Mara River will be crowded... they're there for the same reason as you.

Numbers did drop on the coast soon after the incidents at Lamu, but have picked up. And, that was mostly at resorts in/around Lamu, but not the southern beaches. For the few properties in the Lamu area that did close, opened about a month after the incident.

In my experience, September and October will be quieter than July and August. However, by October the migration may have returned to Tanzania.

I have spoken to friends in the industry, and bookings are looking strong for the high season. Pretty much from early July through to mid October. There are quieter pockets, but still looks strong.