这在ggplot 3.4.0（November 2022）中发生了变化。如果你搜索这方面的问题，你会看到很多使用stat()或..count..的代码。
但是，这在3.4.0中被弃用。现在可以使用after_stat(count)来计算计数：

ggplot(df, aes(x = population, color = Smoking, fill = Smoking)) +
    geom_bar(position = "dodge") +
    theme(axis.text.x = element_text(angle = 90, vjust = .5, hjust = 1)) +
    geom_text(
        stat = "count",
        aes(
            label = after_stat(count)
        ),
        position = position_dodge(),
        color = "black",
        size = 8,
        vjust = -0.2
    )

我喜欢得到geom_text内的计数，就像@SamR的答案一样。
另一种方法是在调用ggplot之前计算计数，然后在geom_text中使用它们

df %>% 
  mutate(count_data = n(), .by = c(population, Smoking)) %>% 
  ggplot(aes(x = population , color = Smoking, fill = Smoking)) + 
  geom_bar(position = 'dodge') + 
  geom_text(aes(population, count_data, label = count_data), 
    vjust=-.5, color="black", position = position_dodge(.9)) + 
  theme(axis.text.x = element_text(angle=90, vjust=.5, hjust=1))