Shihan is an organizational title more than an honorific and different organizations use it in different ways. A member of my organization would use the title to refer to our head but we would not be required to use it for a different organization. I tend to use it for Aikikai and Judo shihan but that is just a personal choice.
In Aikikai (in Japan) there are quite a few whereas in Shodokan Aikido there are only two. There would be more in the latter organization if they used the same criteria as the former. In Judo, Shihan is tied in very closely with rank (6th Dan) whereas in Aikido that is not necessarily the case.
Hope this helps.