Within the field of academic history, we have more historians (by which I mean fully qualified PhDs) than we can employ. Last year, we produced twice as many new PhDs as there were academic job openings. That means that many historians are only able to work part time in the field, or on short-term contracts (about25% donthis), and many have to leave history entirely and work in other fields (like government, or non profits - another 25% or so) (source).
However, not all specializations are equally flooded. American history is glutted with too much tallent, and we can't begin to employ everyone who enters the field. European, Latin American, and the Middle East are alright - still too many, but less of a crisis than American history. The real growth on the market are Asian, African, and possibly also World history. This article compares recent graduation numbers (of PhDs) with job openings. In a selarate article, Figure 3 shows what specializations have been growing for the past several decades, as reflected in departmental hiring.
If you're looking for guidance to prepare yourself for the market, you have to approach these numbers with care - it takes, on average, just under ten years to get through graduate school in the US (incl a masters), during which time a lot can change - Middle East looked hot ten years ago, but has now levelled off, perhaps because of the number of people who jumped on the wagon post 9/11. The interest in World History (or Global history - history that crosses geographic and political boundaries) may or may not prove to be a long-term hiring focus of departments. It's also not clear whether European history is still going to decline as departments correct for a generally shameful neglect of Asian and African history, or whether we'll see (or have already seen) things level off. In addition to the articles linked above, this provides some good (but slightly older) data and discussion of the rebalancing of departments to include a more global focus.
So: where do we need historians? If we're talking PhDs, we don't. The market's flooded. But your odds of getting a good academic job increase dramatically if you study Asia or Africa. You'll do alright in Latin America. You can make it work in European history. Do a PhD in American history if you're willing to work outside your field once you've finished it.
Now if, on the other hand, you just want to get a history BA, or to study history casually but not pursue a PhD, all this advice becomes less important. The skills you get from a history BA translate widely into a number of fields that value the ability to research, to analyze texts or data, to present that analysis using clear and cogent writing, an understanding of and abilitybto articulate how humans and human societies work, and experience studying and understanding different cultures and traditions. A BA in history allows you to be pretty creative, so don't feel like you have to pidgeon hole yourself into a gap in the PhD market that I've described above.
waves hand
At least in the United States and the English speaking world, Byzantium is woefully understudied. Many of the most important primary sources remain untranslated into English. It makes it incredibly hard for people like me, undergraduates trying to write but who can't read Greek, to do work when the guy you want to write an article on, John II, is mostly recorded in sources that are untranslated. My school, the University of Michigan, doesn't have a dedicated Byzantinist and rarely offers a Byzantine class. Antiquity in general doesn't get the attention it deserves, surprisingly.