Every astronaut launching on a Soyuz has a custom-made seat. The seat should prevent injuries from the very hard landing. Are there any numbers about the maximum decceleration when hitting the ground?
Landing deceleration for the TMA version is 5G. The 12G value I'm not sure of, that could be the max. value for a ballistic reentry (so would occur high in the atmosphere) - the wording is unclear.
I'm late, but nevertheless.
I am sure the seats are custom-made mostly for land contact, not atmospheric reentry. The accelerations are far beyond 10 g in that moment, although brief. Some astronauts compared Soyuz land contact with a "car crash".
Solid-fuel rockets soften this at last moment, but not so much. Also in early Soyuz flights there were two incidents the rockets did not fire, so the landings were even more hard. The option is still survivable but injuries are posible. So the custom-made seats lower the injury chances.
According to the book by general Gudilin the maximum accelerations were about 30 g for Soyuz-35 in June 1980 because of landing rockets failure.
при посадке корабля "Союз-35" (Кубасов В.Н., Фаркаш Б.) не произошёл запуск двигателей мягкой посадки. Энергию удара восприняли амортизаторы кресел. Перегрузки на экипаж были предельно допустимыми (около 30 ед.). Анализ показал, что система "Кактус" (посадочный высотомер) не выдала команду на двигатели.
during the landing of Soyuz-35 spacecraft (cosmonauts V.N. Kubasov, B. Farkash), the soft landing rockets did not start. The shock absorbers of the seats took over the impact energy. Accelerations on the crew were the maximum permissible (about 30 units). The analysis showed that "Cactus" system (landing altimeter) did not issue a command to the retrorockets.