If it's you, your immigrant husband, your mother and father the answer is 4. If others live there, depending on who they are, they could be included too.
If your father is dependent, 4.
If your father is independent, 4 but you could have him ALSO be YOUR CO-sponsor and file his own I864a and be included on YOUR I864 and then all three of you could COMBINE (CO-sponsor means COMBINE income) income to meet the FPGLs for 4.
Seems like you said Mom meets it for 4 already though in your original post so you don't need to use Dad at all (he'll be included only as her spouse and not a sponsor)???
Once your timeline is filled up I'll help some more.