Shibuya to Narita via NEX

If I intend to take the NEX train to the airport at the scheduled 14.46pm time, should I reserve the ticket way in advance to ensure I get a seat, or is it okay to reserve a few hours or a day ahead?

The Narita Express is virtually always almost empty, so even booking a few minutes before the departure should be fine. Of course, booking long in advance never hurts, however. ;)

is it okay to reserve a few hours or a day ahead?

When in a year will you travel?

Your station in NRT is either Airport Terminal 2 Station or Narita Airport Station. (Narita Station is outside the airport.)

In Shibuya Station of JR, Yamanote Line platforms (Tracks 1 and 2) and the other platform (Tracks 3 and 4) are located apart above the ground. Narita Express trains use the latter, and New South Gate is close to it.

The Shibuya railroad terminal consists of a few buildings with tracks of the four railroad operators, which lie on the 5th basement to the 3rd floor above the ground. Areas around Shibuya station buildings have been undergoing redevelopment. I recommend you would check your route to JR Shibuya Station by yourself, not relying on old guidebooks.

as mentioned before the nex is practically empty heading to narita, but you don't really want the hassle of getting your ticket moments before. It's best to book it a day or so in advance.

Thanks everyone. Travelling in November but I will check the schedule in case of any changes to the timetable. I suppose I should be cautious and book the morning of the return journey just to be safe.