It is possible to lose an entire brain hemisphere and retain complete functionality (see https://en.wikipedia.org/wiki/Hemispherectomy).
There is a huge amount of redundancy in the brain, and most brain matter is only concerned with I/O, signal processing, and life support. It's one of the reasons I have a lot of hope that cryonics is feasible -- massive loss of brain tissue need not mean irreversible loss of an individual.