Technically its unlimited although its rare to see max vertical wind shear component any higher than 9. If using Jetplan it calculates the wind shear based on a formula that samples wind velocity and direction two thousand feet above and two thousand feet below the cruise altitude.
Vertical wind shear values should only be used for guidance as they do not necessarily highlight areas of CAT. Better to use high level sig wx charts and PIREPS for this.
Buzzz