So how did you find out how many seats were available?
Notice I said "is available on MAV". I put it that way because I don’t know if OBB and MAV both sell all the same seats. I suspect yes, but without comparing the two, I don’t want to say what I am not certain of. But to answer your question, I just went through the booking process on the MAV site. You will reach a point where you get some options, and one is to see a graphic plan of the carriages and you get to pick a seat just like on an airline booking.
For the train in question MAV will sell you 2nd class seats in carriages 22, 23 and 24 which have available 3, 58 and 21 seats [the carriages have between 70 and 75 seats each]. Two other carriages are shown but not selectable which makes me wonder if those you can buy from OBB.... don’t know. But one of those only has 4 seats. Oh, on MAV you will be charged in HUF, the EUR price is approximate, so the fare is 8.400 ft and the seat is 800 ft, so, 9.200 ft total (today about $27.00 or 24.00€.)
Okay, I looked at OBB, because I was curious about seats. To start with that 24.00€ ticket at MAV will cost you 55.30€ with the seat reservation on the OBB site. Interesting that OBB charges more for the ticket and more for the seat reservation than does MAV. OBB offers seats in carriages 21, 22, 23 and 24 which have 56, 0, 58 and 18 seats. So, OBB has one more carriage with seats available. Sounds like quite a few seats are available. MAYBE.
I remembered a few of the Germanic locals on the forum insist that locals only buy their tickets the day before, or even at the train station, because the trains never sell out and presumably they don’t mind the higher price. So I looked at tomorrow’s 9:40am train to Vienna. There is one seat available on the OBB site. NO, this is not a sell out because OBB will keep selling tickets to those who want to stand in the aisles and beat those in the seats with their backpacks.