https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/in-depth/

Ceres could be up to 25% water.

https://www.usgs.gov/media/images/all-earths-water-a-single-sphere

An asteroid (well, it'd be a dwarf planet) composed only of water-ice with the same mass of Earth's water would, presumably, be slightly larger than those spheres. Because water expands as it freezes and a dwarf planet would be at least partially frozen.

https://upload.wikimedia.org/wikipedia/commons/1/16/Ceres_Earth_Moon_Comparison.png

So, looking at the size of Ceres compared to Earth/Earth's water, I'd assume that we'd only need 5-6 bodies similar to Ceres to fill us up.

https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/

They think there is more than twice the amount of water on Europa than on Earth. Not even mentioning the other very wet moons of the outer planets. The article here believes Earth water came from the outer solar system, and, looking at all the information here, it should be easy to see that some asteroids are basically water balloons. Even if they aren't, there are still plenty of water rich bodies the Earth could have amassed.

edit: typos