I suspect that it is 1 attack per turn. However, I think FFG may have missed a great opportunity for game play. They need to add Gunner cards. If Y-wings had a gunner icon, then they could have a Gunner Upgrade card which would represent a backseat gunner. They still could salvage this since the Battle of Yavin was mostly Longprobe Y-wings I believe that do not have a gunner. A future expansion Y-wing could be a two seater and allow Gunner cards. I think it would be great to have personality cards for the gunners as well. This could also work if they ever add the Falcon. Han Solo pilot vs Han Solo gunner cards.
The clone war Y-wings would certainly have a gunner. The EU also have several other fighters with gunners I believe. Hmm, the possibilities are very cool.
According to Expanded Universe canon, there are at least four Y-wing models.
BTL-B: The Clone Wars model. Heavier and larger than later models, he is a two-man fighter armed with a bubble turret and it is more focused towards making torpedo runs against capital ships.
BTL-S3: A Rebellion Era model. A two-man fighter-bomber equipped with an ion turret.
BTL-A4: A Rebellion Era model. A one-man fighter with the ion cannons fixed towards. It is the model seen in the Original Trilogy models. I suppose that the pilots of Gold Squadron surely missed the 360º turret during the Death Star Trench Run in the Battle of Yavin.
BTL-A4 (LP): The "Longprobe" model, an scout one-man fighter equipped with better sensors, more fuel and a more powerful hyperdrive. This model could be seen (and flown) in the extinct "Star Wars Galaxies" MMORPG. This was the model used in that game: http://images1.wikia...4/Y-wing_LP.jpg
We can assume that the standard Y-wing of the "X-Wing Miniatures Game" without the ion turret upgrade is an A4 model, and the ion turret equipped is a S3 model. We could argue that the A4 model still has the ion cannons, and it should be able to fire them forward, although some Expanded Universe sources say that many A4 models replaced the fixed ion cannons with blaster cannons, and that would explain the ion cannons absence in the standard Y-wing.
About the possibility of having "gunner characters", I found it highly unlikely, as the only "character gunners" that appear in the movies would be Luke Skywalker and Han Solo when the man the Falcon' turrets during the Death Star escape. Skywalker is already represented as an X-wing pilot without referring to an specific squadron (he could be Red Five or Rogue Leader), so I don't think that we see alternate version of a same pilot (the same can be said about Wedge Antilles, he appears as Wedge Antilles, not as Red Two, Rogue Two or Rogue Leader). BTW, I also think that the "Red Squadron pilot" card already covers all the rest of pilots of this squadron, so we don't see a Jek Porkins card, and any future ace X-wing Rebel pilot card wills come from the Rogue Squadron (like Zev Senesca and Hobbie Klivian).
PD: The text of the "Fire the ion cannon" news update confirms that the basic Y-wing is an A4 model. An the generic pilot cards seems to be "Gold Squadron pilot" and "Grey Squadron pilot". We know that one of the ace pilots is "Horton Salm" (Grey Leader during the Battle of Endor). I cannot make the letters of the name of the second ace, but probably it is some Gold Squadron pilot. My bets are on Davish "Pops" Krail, Gold Five during the Battle of Yavin (the last to die during the Y-wing trench run, his last words being "They come from behind!").