There are no exact statistics for dead altcoins. altcoins will die because of the lack of interest for these altcoins, usually this happens in new projects, I say this because fraud often occurs in new projects, so many investors are traumatized and make them reluctant to invest again. speaking for the top 20 coins is impossible to lose, they have progressed well and also have a solid team.
Altcoins that die are usually the result of a loss of care on the part of the team itself so that its development stops and the enthusiasts disappear in the market, so the life of such coins will not last long to survive, and those in the top 20 cryptocurrencies are clearly solid because if they are not solid, there is no way that altcoin could ever be in the top 20.