AT(admission ticket 入場券 nyuujou-ken) won't be sold out, no worry,
you can buy it at regular ticket vending machine anytime like this.https://www.youtube.com/watch?v=Mia2bE3NMLk
AT fare is 140Y/adult, valid for only for 2 hrs(120 mins), not allowed to enter train(cars).
It is necessary even if you enter Shinkansen area fr JR-EAST conventional line train.
AT is only a paper ticket, IC card ticket(such as Suica, PASMO) cannot be used as AT,
The foreigner in the above video can read Japanese,
he purchased fr Japanese display menu but English button with, easy.https://www.youtube.com/watch?v=u5Q-1hUXKrQ
Also he used IC card Suica instead of cash, but this way is not so important.
And,,, JR TOKYO stn is a bit special as track layout,
Tokaido Shinkansen where DY arr/dep is "JR-CENTRAL(JR-Tokai)'',
so you cannot see DY closest if entered JR-EAST Shinkansen ticket gate,
but with same AT, you can also walk around freely
to JR-EAST Shinkansen and conventional line(such as Yamanote line) platforms.https://global.jr-central.co.jp/en/info/station/_img/tokyo.jpghttps://postimg.cc/6y7wdBfQ
At that time DY would be arr/dep fr No.19 track, the easternmost, if not changed.https://www.youtube.com/watch?v=41ooWsjsWCQ
DY cannot run on JR-EAST Shinkansen tracks, white/red shinkansen "East i" works, anyway.https://www.youtube.com/watch?v=VW2zjtHlgGw
"Color" is the biggest reason why DY is more popular than "Eeast i".https://www.imdb.com/title/tt0076935/
Real Japanese rail fans like both, though.
Ticket gate inside area, you can not only buy EKIBEN(駅弁 box lunch)https://www.youtube.com/watch?v=DV7nDWMClE4
but also many shops such as cafes, restaurants and more.
This video started B1F Yaesu gate(East side) area, the West side called "Marunouchi".https://www.youtube.com/watch?v=S5Lb2uUu3UYhttps://en.wikipedia.org/wiki/Tokyo_Station