I want to Extend stay for my Father and Mother who are on B1/B2 visa. I can fill out I-539 form from USCIS to renew. I was confused if I have to fill two I-539 forms for each of my parents or just one form for I-539 and and spouse form I-539A. The instruction url for I-539 is 404 (file not found) so I am not able to get more information. Can any one of you tell me who renewed their parents B1/B2 visa recently ?

https://www.uscis.gov/i-539