You don't actually have to count every bar that you don't play especially if you go many bars without playing anything. Most large form pieces and even Jazz pieces are broken down into smaller parts i.e. an A section, a B section, C section, ect. So you could have sections A, B, and C be 32 bars each and you many not start playing until 4 bars into section C and instead of just counting 68 bars you could listen for the start of the C section and then count 4 bars.
If you know the song inside and out you may not even need to count the 4 bars you may just be able to "feel" when you come in.
Also the conductor in an orchestra will cue the instruments coming in as it is what they do.