Does extension through i-539 grant 6 months of extended stay for b2 visa or the last date of stay is the new departure date filled in i 539?

Unfortunately, the immigration law only gives you what you ask for, and in this case your parents asked for five months instead of six. Under the rules of immigration, they are allowed to remain during the time that their application is being adjudicated, but when approved, the approval would only be for the five months and they would not be in status when their applications are decided. I do not know whether a consular officer or immigration officer at the port of entry might consider that they violated status and that their visas are no longer valid for travel. Hopefully any adjudicator will take cognizance of the difficulties of travel during the time of pandemic. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.