You can find this information on the USCIS web site. I googled "stay in us on b1/b2 while extension pending" and this was the first hit, from an actual official source rather than some random person like me:
https://www.uscis.gov/sites/default/files/document/guides/C1...
And look -- the PDF has the answer:
> If I am eligible for an extension of stay and file on time, will my stay be extended?
> An extension of stay is not automatic. We will look at your situation, your status, and the reasons you want to extend your stay before deciding whether or not to grant your application. If we grant it, we will also decide how long to extend your stay. We will not grant an extension if circumstances indicate that an extension is not warranted.
When dealing with USCIS best to go in person and get an actual decision in writing, because the downside of taking advice from web sites and even USCIS documents can get severe.