For a community tank, I'd recommend 4 as the minimum number to keeping cories.
10 gallons will fit 3-4 large species of cories. 6 minimum for panda and other pygmy types of cories.
If you want to breed them, the sex ratio is two males to one female as opposed to the sex ratio of livebearers which is one male to 2-3 females. The reason is that in cories, their style of breeding is quite different. The female chooses which male she wants to help her with the laying process.
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