How to counter a team of multiple Torbjörns?

A Reinhardt or more than one, with good long range damage dealers such as Widowmaker, Zenyatta, Soldier: 76, or Pharah. The hard part is getting everyone to hide behind Reinhardt's shield, but once everyone's in place, the turrets deal very low DPS and will be easy to take out.

From my own experience, a Reinhardt or two is almost completely necessary. I would recommend throwing a Zenyatta in as well; despite being squishy, his orb of discorb is a huge help against Torbjorn, especially when he's pulled his Molten Core. The key to pushing past a Torbjorn defense is to kill him, that way, he cannot rebuild his turrets. A damage debuff is the key to quickly eliminating them.

As other answers have stated, snipers and the like (Pharah, Soldier: 76, etc) can all assist, and the absorb/reflect of D.VA and Genji is invaluable, but the debuff gained from Zenyatta's orb can make things all the more easy.

EDIT: I've often found that as soon as a wall of Torbs'n'Turrets is broken, the point is usually yours. The turrets are only truly dangerous as long as Torbjorn is still around. If you kill all the Torbs, they aren't likely to be able to setup a new defense on that point.

As several have said already, Genji, Pharah, Soldier 76, Zenyatta, and Reinhardt are all good picks against a team that has one or more Torbjörns. However you should also look at D.Va, Winston, Hanzo, Widowmaker, Junkrat, Tracer, Zarya, McCree, and Mei. All of these can help destroy turrets in the following ways.

• Genji can deflect turret damage back at the turrets to kill them.
• Pharah has long range damage with her rockets that do splash damage, so she can hurt the Torbjörn who is repairing the turret at the same time as damaging the turret. She can also knock the Torbjörn away from his turret with her E ability, and if someone is distracting the turrets, her ultimate can quickly clear out turret nests.
• Soldier 76 has good dps that is quite accurate at mid range, and a full clip or two into helix rockets will generally kill a turret.
• Zenyatta while being a support, does a lot of damage with his primary and alternate fire. If he is shielded, he can not only heal people who get low, but charge his alternate fire to help burst turrets down. He can also use transcendence to keep his team alive while pushing in to clear turrets.
• Reinhardt is pretty essential for killing turrets. While they may chip his shield down fairly quickly, he can provide protection for multiple members of his team who can all provide damage to destroy the turret.
• D.Va can use her mech to absorb a lot of damage from turrets, as well as completely negating a turrets damage if she uses defense matrix on it, allowing her to distract turrets for her teammates to kill. Also, when she gets her ultimate, she can easily clear a room or point where the turrets are setting up by boosting into an eject and letting the mech drop in on the turrets.
• Winston can thrown up a shield around a turret and completely prevent it from shooting his teammates outside. Also, his tesla can hit multiple targets which can help clear multiple turrets at a time. It may be a good idea to run multiple Winstons to counter multiple Torbjörns.
• Hanzo and Widowmaker can shoot down turrets for long range. Without getting hit by the turret. Hanzo has the added benefit of being able to arc arrows so he can hide at a slightly lower elevation and still hit the turrets.
• Junkrat can get his primary fire into hard to reach rooms to clear out turrets inside by bouncing them off walls. He can also arc his shots over walls to clear out turrets behind a wall. His ultimate can also quickly kill Torbjörns and their turrets if it isn't destroyed.
• Not as obvious, but Tracer can dart around a wall and get damage down on a Torbjörn or his turret and recall to safety. Doing this can eventually wear them down, and with ultimate up you can drop that on a Torbjörn to instantly kill him.
• Zarya can put up a barrier on whoever is being targeted by the turret and gain free charge while providing protection for his teammate. She can then used her super charged weapon to quickly damage turrets and destroy them.
• McCree, if he can find a good flank on the turret, can actually flash bang the turret so he can flash bang into fan the hammer for some good damage on the turret. He can kill the turret with this combo if he lands all of his fan the hammer shots.
• Mei is also a great counter to turrets, as she can wall them off to prevent LOS so that the turret cannot fire on her team. She can also freeze turrets with her primary fire or her ultimate.

Note: You should always have a healer to heal anyone who is taking damage while taking out the turret. Mercy can also damage boost people who are safe from the turret damage in order to more quickly kill it, and Lúcio's sound barrier can help negate most of the damage and give people enough health to kill the turrets. Sound barrier and resurrection can also help counter Molten Core.