At a concentration of 20,000 Units per ml is 20 Units per microliter. And the definition of a unit is usually the amount of enzyme needed to digest 1ug of DNA in one hour. So to digest 100ng of DNA you would need 0.1unit or 0.005ul to digest in 1 hour. So if you add 0.1 or 0.2 ul of enzyme you should be fine.