The "rules" are that when you change a topic, you change paragraphs.
In reality, when a paragraph gets to be quite long, you should look at splitting it, for easy readability. You should split it logically, and consistently, however.
In your case, if your information on each country will include several sentences covering several areas of interest in that country, you might split it into paragraphs for each area of interest: political, social, economic, history, etc. that may have contributed to the genocides. Trying to do the same for each country will probably help you organize your work better, as well. You won't forget to mention the history of one country, if you know you have a paragraph about it for every other country.
You can also use subheadings in most works. I would check with the style sheet for the institution that you are writing for (school? magazine?). They may have a preferred style. If you can use subheadings, then each country is probably a sub heading, then a very general paragraph introducing that country or comparing it to the others, followed by paragraphs on each of the areas of interest (history / politics / etc) in that country.
A paragraph can be as short as one sentence, but in a scholary work, you should have a little more depth. Start with a topic sentence, then add other sentences to support what you have just said. That's support, not repeat. Some will suggest you should end then with a short summary sentence of the paragraph, but I find that usually is contrived; when you run out of support for your topic sentence, just start a new topic with a new paragraph.
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