Specifically, seeing the number of historians underneath a given topic, era, geographical location, or theme, can this be taken as at all representative of the saturation level of people within a given field? Meaning, there's a glut of historians here representing European history and a comparably very small group representing Middle-Eastern history.
Is it reasonable to expect that this generally reflects the "industry" as a whole? To be blunt, would it be reasonable to look at that as a rough guide to which areas of history would be easier to get into, career-wise?
I would say, it's not at all representative.
An actual degree isn't required for flair. Self-study is considered an acceptable equivalent to a degree. All you really need is a demonstrated understanding of a topic, an ability to write comprehensive answers, and an ability to understand and properly cite the sources for your speciality. Many of the flared users don't have a history related degree.
Of course as this is also an English language forum, so our users tend to focus on subjects that touch on British and American topics. We have an excess of WWII experts and a dearth of say, Latin America experts. Or as you say a lot of European medievalists and relatively few Middle Eastern medievalists.
You'd be far better served to look at faculty lists at universities like this one from Harvard to get a feel for what is more representative for the "industry."
You may also want to consider languages when considering a specialty. You're going to have a hard time if you can't read the primary sources. While there may be a lack of Middle Eastern medievalists, you're going to need to pick up Arabic, and possibly Persian, Greek, Latin, and maybe something else more obscure (Coptic perhaps?) to really become a specialist.
There may be a lot of experts in 19th century Americana. But there's also a ton of accessible sources. For an English speaker there's a lot more possibility to do original research.
If we compare the flair list with this report by the American Historical Association from 2011, a few things stand out.
First, European flairs appear to be over-represented in AskHistorians. The AHA study shows degrees in European history being the 2nd most numerous, with about 25% of new hires between 2000 and 2010 being in this field. Compare that with AskHistorians where out of 371 total flairs (not including Quality Contributors), 119 are European flairs, working out to roughly 32% of all flairs.
On the other side of the coin, US flairs seem to be under-represented with only 41 flairs (~11%) being for North America, while US history degrees from 2001-10 represented approximately 38% of new hires in American colleges and universities.
Of course, AH has lots of flair divisions that aren't really covered by the AHA study. Military flair is the 2nd most common in AH, with 50, but the study doesn't specifically mention military history*. Art history, Science and Technology, and Religious history are also AH fields but are not categorized separately in the AHA study.
Furthermore, when you get to specific flairs, the question is further complicated. There is a section for Archaeology flairs, as well as individuals like /u/commodoreCoCo and /u/ucumu who are archaeologists with Middle and South America flair. There are also linguists like /u/rusoved, /u/keyilan and /u/limetom. Also, there are users like /u/anthropology_nerd and /u/firedrops who are sociocultural anthropologists. In a university setting, these individuals might be hired into anthropology departments rather than a History department proper.**
Similarly, many universities have Classics departments that are separate from their History departments. A fair number of our Europe and Military flairs specialize on Roman or Greek social, legal, and military history, and could conceivably be categorized as Classicists (though Rome and Greece do exist within history departments as well).
On the other hand, Africanists, Asianists, Latin American and Middle Eastern flairs all seem to be under-represented in AH when compared to the results of the AHA study. I would also point out that when jobs in Area Studies departments (i.e. African studies, Middle Eastern Studies, Latin American studies) and even anthropology departments (African archaeology, African anthropology, etc) are considered, a person with a passion for one of those fields and a desire to get a history, anthro or area studies doctorate would have slightly more career openings than the number of flairs on AH would indicate. The trend line from the AHA study also indicates that these positions are increasing as a percentage of history departments (though who can say if the trend from 2010 still will hold in 2020 or 2030?)
*My personal opinion is that the demographics of the subreddit (overwhelmingly young and male) means military history is over-represented as a number of AskHistorians flairs, and that college/university jobs in military history would be rarer.
** Any anthropologists reading this, please don't read me as saying you "don't belong here". I am very happy that AH is able to include specialists of many disciplines, showing that there are multiple ways of arriving at knowledge of the past.
EDIT- I forgot to mention one important matter. Askhistorians serves a multinational audience. We have plenty of European, Canadian and Australian flairs and users. That AHA study only reflects American history departments. OP, if you are in Europe or Asia or Australia, the situation might be entirely different.
Um. The first time this happened, I assumed I did something wrong, so I deleted and reposted. Now it's happened again. I have no idea why this is getting highlighted and flagged under this week's theme...