JR West Online Reservation

hello, I made a reservation through WESTJR website for tickets for Tokyo - Nagano line.
I booked two not reserved seats to be paid at the station and it's says "Your reservation has been accepted"

Since this will be during golden week (May 3rd) and lot of trains are already sold out do I have to go to the ticket reception office few days before or my reservation is a sure thing and I can go there like one hour before the train?

Your question is very confusing! You speak about reservations, but in fact you purchase a non-reserved ticket? If you made a seat reservation, then your seat reservation is guaranteed. If you purchased a non-reserved ticket, then you won't have a seat reservation. You can board the non-reserved cars and compete for getting a seat, but it won't be guaranteed.

The trains in the direction of Nagano are going to be super-cowded in the evening of May 2 and the morning of May 3. The situation should gradually become better towards the afternoon and then the evening of the 3rd.

Pretty sure those tickets need to be collected at least one calender day in advance.

There is no need to pick it up a day in advance, but you will have to pick up physical tickets, as explained on the JR West website.