Question about distance from subject

Okay so here's my question.  I understand aperture, ISO, and shutter speed.  But I am trying to figure out is distance from my subject.  For example, if I'm shooting a family of 4 at a f/1.8 ; how far away from them do I need to stand in order for them to be in focus?

Also, what the ideal f/stop for shooting 1 person, 2 person, 3 persons etc- so everyone is in focus?  I seem to always be getting someone out of focus in my photos at a low f/stop.  I have see DOF calculators, but they just don't make much sense to me.

Today, I am shooting my first maternity shoot and want to make sure that her whole body is in focus... so would I use the focus points and JUST focus on her face or would I use the focus points that focus on all of her body??

Thanks so much!!