It really does depend on the parts of the brain damaged, and the severity.

A person can completely shift in behavior post tbi. And it absolutely can cause damage that impairs learning and thinking.

It’s impossible to give real numbers because the last time I went looking, there wasn’t enough research out there to properly break down the possible outcomes. That being said, I had a decent share of tbi patients over the years. That fact skews things, obviously; if they needed a caregiver, they had a more severe injury in the first place. But personality shifts were common, and functional capability varied greatly, even within the type of injuries that required caregivers after the fact.

It’s frankly terrifying to run into the patients that are 100% there intellectually and awareness wise, but stuck with limited ability to communicate.