library("ggpubr")
ggscatter(mtcars[mtcars$cyl == 8, ], x = "mpg", y = "qsec",
     facet.by = "cyl", add = "reg.line", add.params = list(color = "black", fill = "lightgrey"),
     conf.int = TRUE, cor.method = "spearman", cor.coef = TRUE, xlab = "mpg", ylab = "qsec")
     + annotate("text", x=10, y=15, parse=TRUE, label = paste0("n = ", with(mtcars, sum(mtcars$cyl == 8))))

label = paste0("n = ", with(mtcars, sum(mtcars$cyl == 8)))