You can put up to eight zebra danios in a 15-gallon tank. As a general rule, one inch of fish requires one gallon of water to thrive, and a fully grown zebra danio is approximately two inches long. Therefore, each zebra danios need two gallons of water to swim freely.
You can thus fit seven or eight zebra danios in a 15-gallon tank. If you can afford to get a bigger tank, then go for it. Zebra danios are very active and love swimming around, so they’ll appreciate having more space to explore.
However, you should note that this general rule is for species tanks only. If you have a community tank, then you also need to consider the other tank inhabitants.