It’s possible to use the option tag in the function labs() for adding label to each plot.

For example, kindly try this:

library(ggplot2)
library(patchwork)
p1 <- ggplot(mtcars) + 
  geom_point(aes(mpg, disp)) +
  labs(tag = "A")
p2 <- ggplot(mtcars) + 
  geom_boxplot(aes(gear, disp, group = gear)) +
  labs(tag = "B")
p1 + p2