NEX

hi
I've been reading about resrevation seats for NEX before you arrive at tokyo. I am confused, why theres a rules that I need to get the ticket days before I arrive at the departure date? How will i get it if we need to get it before we arrive in japan?

Here is my situation... we are 8adults with 3 child below 5 yrs old. We will arrive january 3 2017.
Our route is only narita terminal 1 to shinjuku station.
Then is there a ticket to buy in advance for subway tickets?
When do I need to reserve our NEX ticket?
Can i get the reserve ticket on the day we arrive?
Im really confused and scared that we might dont get any seats available if we purchase on that day.