There is no single answer to how many international units (IU) are in 1 millilitre (mL), as it depends on the substance being measured. For example, 1 IU of insulin is equivalent to 0.0347 mg, while 1 IU of vitamin D3 is equivalent to 0.025 mcg. Therefore, the number of IUs in 1 mL of a substance will depend on the concentration of that substance in the solution.