JR car numbers and correct platform location

Would appreciate help in explaining how to interpret car number on JR reserved seat ticket in relation to correct door to wait for train. Sometimes there are several different shinkansens noted on the floor or above signage but not sure which door we should stand. Often we find ourselves asking for help to be sure we are standing at the right door as some trains have fewer cars. Thank you.

There are only 2 doors per car and you can enter through either one. The electronic signs above show where the cars of the upcoming train will be. I'd use those signs if the floor ones confuse you. There is no clear way to explain it other than seeing what train service is on the ticket and matching it with the one on the floor sign. If you have any problems, ask the attendants on the platform.

No entering rule closest door in possible
if no time on platform or not.

(note)
Some 2 trains as 1 type(not only shinkansen)
cannot walk thru each trains by head(front car) to head until next station stop.
http://blogimg.goo.ne.jp/user_image/32/20/188fd9f60f8dae82da7313d1299c...

The number of cars you train has is indicated on the display panels (along with its destination, departure time, etc.).