You have no way of knowing. It all depends on what other teams do this season. I would say they wouldn't get in over a 1 loss Ohio State, Alabama or Oregon team this coming year. In fact FSU with 1 loss wouldn't get in over any 1 loss team from the SEC or Pac-10 due to the strength of those 2 conferences. I would say if FSU finishes 12-1 there best shot is probably that #4 seed. That isn't 90% chance.
The other problem is when does that loss come and to whom? FSU's schedule is likely not strong enough this year to lose a game and still look like a top four team.
-If they lose to Clemson, Clemson probably wins their division in the ACC. End of chances as FSU would not go if they don't make the ACCCG.
-If they lose in the ACCCG, they wouldn't go with a loss on the last weekend.
-If they lose to Texas State, USF or Chattanooga - losing to any of those teams would automatically kill their chances. Same with BC, WF, NCST and Syracuse as each will have at least four losses, most likely even or losing records.
That leaves GT, UL, UM and UF. Which of those four are going to be a two loss or better team?
-If its Louisville, they probably win the ACCA over FSU, so that is moot like a loss to Clemson.
-Miami? Nobody expects Miami to lose two or fewer, so a loss to UM at home would look too bad.
-UF? Few people expect UF to lose two or fewer. A loss to anything but a 0 to 2 loss UF team that late in the season ends their chances. Even if UF had that good of a record, it would still be questionable if they get in because they would likely have lost to the best team on their schedule and would have one or fewer wins against a quality team(Clemson or UL).
So this essentially leaves GT. If FSU won every game except against GT, and then beat GT in the ACCCG, then they might get there. That's about the only realistic scenario where a one loss FSU makes the playoff. But even then, it would depend on what teams in other conferences finish.
To sum it up - if FSU loses to any team other than another playoff contender, they will almost assuredly never get into the playoff with one loss(not just this year, any year). Even if the loss comes to such a team, it is still not guaranteed considering they probably lost to the best team on their weak schedule and/or in the last two weekends. And this pretty much goes for any team in the ACC as long as the conference is seen as the 4th or 5th best conference year in/out.
But most likely, a depleted FSU squad missing the best QB in their history, missing the best TE in their history, missing the best OL they have had in a decade and returning a questionable defense loses three to four regular season games. Its silly arguing about "what happens if FSU goes 12-1?" when its highly improbable that occurs, but what the hey, its funny watching them feign offense that someone thinks a 12-1 FSU team wouldn't make the playoff.