You could create an annotation with a label. Make sure to create some space by changing the plot.margin and clip off the coords. You can play with the settings. Here is a reproducible example:
library(ggplot2)
ggplot(df)+aes(x=a,y=b,fill=b)+
  geom_bar(stat='identity') +
  coord_cartesian(clip = "off") +
  annotate("label", x = Inf, y = 20, hjust = -0.1, label = "TOTAL = 60", size = 3) +
  theme(plot.margin = unit(c(1,5,1,1), "lines"),
        legend.position = 'none') 

Created on 2022-12-12 with reprex v2.0.2
You could use richtext when loading ggtext to rotate an annotated box like this:
library(ggplot2)
library(ggtext)
ggplot(df)+aes(x=a,y=b,fill=b)+
  geom_bar(stat='identity') +
  coord_cartesian(clip = "off") +
  annotate("richtext", x = Inf, y = 20, vjust = 2, label = "TOTAL = 60", size = 3, angle = 90) +
  theme(plot.margin = unit(c(1,5,1,1), "lines"),
        legend.position = 'none') 

Created on 2022-12-12 with reprex v2.0.2