How, exactly, is an accurate count done? I just did a web search to learn more about the migration site and found a photo of a large number of birds in the sky, but how is it possible to determine the species and the number when there are so many at one time? And are there territory issues, or is it not a problem because they are all flying through so, therefore, not in anyone's territory? 
The observers at these sites are a) very skilled at identifying birds as they fly by and b) very skilled at counting large numbers - which is a bit like cooking or maybe baking is a better analogy. Everyone knows someone who doesn't measure when they bake/cook and their food always comes out head-and-shoulders above those of us who always measure. For hawk site observers and for researchers doing surveys of species like waterfowl, you count out a set number of birds - 10, 25 whatever and then you use the physical amount of space those birds take up to let you quickly "count" the rest of a flock or kettle in the case of raptors. And it is a surprisingly accurate way to count birds - the margin of error for experienced counters is very small, statistically irrelevant in fact. So counting 15,000 MIKIs is a question of experience and technique. And they get a fair whack of practice - Turkey Vultures and Broad-winged Hawks also tend to migrate in alarmingly large numbers.
And no, no territorial issues. Territorial conflict is primarily a nesting issue - want territory to nest, have territory and am nesting or have pre-fledge age chicks on territory. After that, the pressure is off for the most part. Doesn't mean there aren't territorial spats outside of the nesting period, but raptors know they can't afford to get hurt so unless there is a real need to defend/offend, they don't. And migrating birds aren't interested in territories or really anyone on territories, they are just going from point A to point B. And migrating raptors aren't spread out over the landscape, they have very specific routes that make use of the land's topography to aid them in their travels, either by helping make them travel more efficiently thereby conserving energy and resources (ridges, coastlines) or ensuring abundant food resources (wetlands, sometimes urban areas) or safe havens for roosting or waiting out weather (urban areas, protected areas). They are much more tolerant of others of their own species - we had 7 adult peregrines on a building one night during migration - and all but one was gone before dawn the next day and none of them were our resident birds and our resident birds were still around and paid no attention. Bald eagles can be very territorial during the nesting season but they will congregate in large numbers on the wintering grounds. And they will often be much more tolerant of other species on their travels, though they can still become lunch to something bigger and meaner - but that isn't territorial, that's just the hierarchy of predation.
Hope that answers a couple of your questions
