B2 visa I 94 expires soon

How soon we can file for extension?

my current I-94 expires on Aug 4th.

please tell me your opinion.

You can consider filing for it like 30 days before. It should be strong reason to extend. If you just apply without proper reason, you may not get extensions for B2.

i am in usa on b1 and my wife and child r on b2 need to extend only 5 days left
1] i tried filing online it takes only my form and says for family fill i 539 a if i fill it how do i upload.
2] If i fill the forms and sent it by post so do i pay for only primary applicant 370 ori have to pay for all 3 370+370 +370 AND BIOMETRICS