Hello,
I am not an expert but I do have 2 Apistogrammas in a 64L tank and they are quite young only 3-4 months and I can only tell you from my own experience and what I can see, there is no way I could add any more to this tank.
They are using all the bottom space and decor to forage the whole day, so I think if I was to add more they would start fighting straight away. I would say start with 2 a male and a female and take it from there.
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