You should keep five to six celestial pearl danios in a 10-gallon tank. Even though they are only one inch in size, each fish requires around two gallons of water.
This is mostly because they require a heavily planted environment. For that reason, extra space is needed to compensate for the area taken up by the plants.
With that said, celestial pearl danios will appreciate more room to explore. These fish can be quite active, and they love swimming and chasing each other around. If you have the resources, aim to house your group of celestial pearl danios in a larger tank.