Your employer can sponsor you for H-1B status for up to 3 years, and apply for a 3-year H-1B extension.
If an I-140 visa petition for a green card has been approved on your behalf under the EB-1, EB-2 or EB-3 categories, and your priority date is not yet current, your employer can apply to extend your H-1B status for 3 years at a time beyond the 6-year limit.
The law also permits one-year extensions of H-1B status beyond the 6th year, even without an approved I-140 immigrant visa petition. These are possible if 365 days or more have elapsed since the filing of either a PERM labor certification or an I-140 immigrant visa petition.