Eight riders pulled out overnight while one of the race favourites, the 2020 champion Tao Geoghegan Hart was stretchered into an ambulance following a nasty fall with a suspected breaks to his leg and pelvis.
Ackermann of the UAE Team clung on to victory as Bahrain rider Jonathan Milan flew level at the line for a photo finish with British 16 time Giro stage winner Mark Cavendish taking third.
"Really, really special victory for me. After my broken coccyx last year, I'm finally back," said Ackermann.
Geraint Thomas of Ineos Grenadiers retained the overall leader's pink jersey but not only lost Geoghegan Hart, but team sherpa Pavel Sivakov, who also went down, limped home minutes behind the peloton.
Jumbo's three-time Vuelta winner Primoz Roglic remains second at 2sec after the 219km run from Camaiore to Tortona, in between Milan and Turin.
Just 141 of the original 176 that embarked on the Giro 12 days ago remain on the race, making it 36 riders out so far, after only 27 pulled out in the whole Giro last year.
Geoghegan Hart was left prone on the ground as several riders came to grief - including pink jersey Thomas and Roglic - on a tight corner 68km from the finish.
Thomas was quick to his feet and after casting a glance at his team-mate Hart, resumed the stage.
Hart - who was third in the overall standings just 5sec off Thomas - received medical treatment before being taken away in an ambulance.
A few kilometres later and Spanish rider Oscar Rodriguez also exited the race after tumbling heavily and requiring attention.
Thursday's stage 12 is a hilly one over 179km from Bra to Rivoli ahead of Friday's run into Switzerland and the Crans Montana in the Alps.
