Why exactly, did Rachel call her son 'Yosef'?
Rashbam: For two reasons: Basically, because Hashem had gathered in (hidden) her shame, only then, he ought to have been called 'Asaf' - so she replaced the 'Alef' with a 'Yud Vav', to hint at the second child that she hoped to bear 1 (or prophesied that she would).
What made her think that she would bear another son?
Seforno: She assumed that she was granted her first son on account of her first Hashtadlus (Refer to 30:22:1:2), and that she would be granted a second son on account of her second Hashtadlus.
Tosfos ha'Shalem (4): The son that was changed inside [Leah] to a girl, was in order that I can have two sons. 3
Tosfos ha'Shalem (3, citing R. Efrayim): She expected to be no less than her Shifchah.
Which explains why she Davened for only one more son.
Perhaps she knew this through Ru'ach ha'Kodesh. Or, the Imahos (Yakov's wives) had gathered to pray for this (Bereishis Rabah Sof 72). Or, Rachel could have known that initially her own fetus was a girl (refer to 46:15:1:1; see also Tosfos ha'Shalem 22:2), and understood that Hashem switched the fetuses to enable her to have two sons (PF).