I think there are an average of 15,000 characters alive at any given moment, being lower at the start of the game. I would guess "in-game" life expectancy to be around 35 years for a character. If a character is created appears 55 and lives to be 60, they are only in the game for 5 years. Depending on the proportion of characters created in adulthood, the "in-game" life expectancy can be significantly lower than the average age of characters at death. Also, the number of characters created is higher than the number of births, due to characters entering the game as adults. Also, the player character is not subject to pruning or the health malice of a courtier, so the average PC lifespan
The game starts with about 10,000 characters. Every year, about 430 are added, or 43,000 a century. The game lasts for 684 years from the earliest start date, so about 294,000 are created in the course of the game, and another 10,000 exist at the start, so I would estimate somewhere around 300,000.