How many grams are there in an ml?

In most cases no. Gram is a measurement of mass (functionally weight) whereas ml is a measurement of volume (how much space it takes up). Depending on density (how much an item weighs compared to how much space it take up) in most cases 1 ml will not be 1 gram. Distilled water was used to make these measurements so 1g of water does equal 1ml of water.