In order to prepare the solution at 20 IU/mL, the activity would be required. The activity of this product is lot specific and reported on the Certificate of Analysis. The minimum activity for any lot would be 150 units/mg. See the link below to review a sample or access the Certificate of a specific lot:
https://www.sigmaaldrich.com/product/sigma/h0878#product-documentation
This material is soluble at 40 mg/mL in water. The same rate would be expected in saline, but rates may vary slightly. Stirring is recommended, however vigorous vortexing is not typically required.
Please see the link below to review the product data sheet which includes information on solubility and solution stability:
https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/743/456/h0878pis.pdf