Since no one decided to answer my question and provide any reference for this number, I did a little research on my own.
Looking at the websites of the major Aikido organizations (ASU, AAA, AWA, Kokikai, Takemasu, Yoshinkan, Yuishinkai, Ki Society) for affiliated dojo lists, I found that there are approximately a total of 801 dojos within these organizations worldwide. I then went to the Aikikai World Federation and looked at two of the larger organizations in there and found that the USAF has 201 dojos and CAF has 67 - this give us a hard fact of 1069 dojos. The Aikido World Federation had another 65 organizations listed, so I gave them a generous estimate of 100 dojos each as an average for the rest of the organization - now we have a very generous number of 8,638 Aikido dojos worldwide within the major Aikido organizations. Of course, we all know there are independents out there as well, so continuing with a generous trend, I gave the same number of 8,638 for the independents; this gives a very generous estimate of about 25,914 Aikido dojos worldwide. Let's round that up to 26,000 for simplicity sake.
OK, we have 26,000 dojos, and still following a generous path, let's estimate we have an average of 25 members per dojo (actually, IMO, I think it is closer to 10 to 15) but I am trying to be as fair as possible here. Doing the math, this only gives us 650,000 Aikido students worldwide - no where close to the 10,000,000 that was thrown out here in this thread. IMO, I think it is really closer to 350,000, give or take a few thousand.