Which is more dense 1 mL of water or 50 L of water?

The answer is: No.

Density is a property of a substance, and doesn't depend on the size of the sample.

Samples of different sizes all have the same density, as long as they're all samples

of the same substance, their compositions are all the same, and the conditions are

the same in every case. (Samples of ice and water have different density, because

the conditions are different.)