Koivaiwouc47 I suggest to reserve seats in advance, some trains on the Arlberg line are very busy. Seat reservations are possible on oebb.at:
https://shop.oebbtickets.at/en/ticket
Select “Find Services” and “Seat reservation only”. For the Railjets you can select the seats on a diagram. It seems that it is not possible now for August, try later.
Some pople have flexible tickets and can use every train. It is unpredictable to know in advance how much people is travelling on a train. Only a ticket gives no guaranteed seat.