Quite a lot,

The science is clear that fast-growing chickens like the Ross 308 are doomed by their genetics. These have been engineered to grow so incredibly fast, and their bodies just cannot handle it.”

Jackson said secret filming at broiler farms supplying big supermarkets has shown birds struggling to walk or collapsing under their own weight, or dying from heart failure, and dead birds were filmed lying among the flocks.

[…]

Andrew Knight, a professor of animal welfare and ethics at the University of Winchester, said: “With these really rapid growth rates, it can be difficult for the heart and circulatory system to keep up with the expanding body mass. A proportion of these animals suffer from heart failure. It’s also difficult for the bones, ligaments and tendons to keep up with the rapidly increasing body mass, meaning that a proportion of these birds become severely lame [inability to walk properly].”

https://www.theguardian.com/world/2022/apr/16/cheaper-than-chips-frankenchicken-at-the-centre-of-fight-for-animal-welfare

They are more likely to have all kinds of other health issues not listed in the quote above such as hock burns