1 l. = 1000 ml.
None, since there is no such thing as a millaliter. Furthermore, marbles come in all sorts of sizes.
The number of milligrams in a milliliter depends on the substance's density. For water, which has a density of 1 gram per milliliter, there are 1,000 milligrams in 1 milliliter. However, for other substances, the conversion will vary based on their specific density.
Not always. The density of water is 1 gram/mL at around 4 degrees Celsius. As temperature rises, kinetic effects kick in causing the volume per unit mass of water to increase, thus decreasing the density of water. Interestingly enough, the density of ice (frozen water) is also less than liquid water. The formation of ordered crystal lattices occupying relatively large amounts of space causes the rise in volume and an overall drop in density.