If we're talking about ethics and academic honesty, you need to make an honest effort to research a good bit.
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If we're talking about good writing, make sure you do enough research to know a few things that would be interesting and fresh to most readers. One of the appeals of historical fiction is seeing something unusual from an earlier culture, especially if the reader learns something.
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If we're talking about getting published and selling books...
You need to know more about the topic than the majority of your readers, and probably more than potential agents or publishers. Any more than that is great, but probably not necessary.
Basically, if your readers aren't going to call you out, you're fine. So it really depends on the subject. Some topics have more critical readers than others.
For example, if you're publishing in the United States and doing U.S. historical fiction:
If your topic is the Civil War, you need to know a lot, because it's an extremely popular area of expertise for amateur historians, and everyone
thinks they know a lot about it. And some people are emotionally invested in it. (Oh, and if you do say something truthful that doesn't mesh with the lies some people choose to believe, you'll still get criticism.)
Same goes for the Civil Rights movements. If you don't portray it accurately, someone will notice.
If your topic is the Mexican-American War, you don't need to know quite as much, because fewer people know enough to argue.
If your topic is the Franklin Pierce administration, you could read his Wikipedia page and know more about him than I care about, and I'm a professional historian specializing in U.S. history.
If your topic is modern transsexual history, you could say pretty much any ridiculous and offensive and poorly researched thing you wanted, because almost nobody knows anything about us. (This is a huge area of frustration for us, obviously.) Sure, it would be immoral and wrong, but it wouldn't hurt your chances of getting published or read, judging by media I've seen.
And as far as the fantasy element goes, most readers expect realism in anything that isn't altered by "magic" (whatever makes your setting fantastic) or obvious genre conventions.
For example, if your setting is "poleis-era Greece with vampires," people might understand the introduction of legendary monsters as being normal animals mutated by vampiric infection. But they won't understand if you don't have hoplite infantry, unless you include an explanation for why vampires existing altered that fact of history.