Question

How can I increase the number of axis ticks in R

Answer 1

You can try to override ggplots default scales by modifying scale_x_continuous and/or scale_y_continuous. 
For example:

library(ggplot2)
dat <- data.frame(x = rnorm(100), y = rnorm(100))

ggplot(dat, aes(x,y)) + geom_point()

And overriding the scales can be done as follows:

ggplot(dat, aes(x,y)) +
  geom_point() +
  scale_x_continuous(breaks = round(seq(min(dat$x), max(dat$x), by = 0.5),1)) +
  scale_y_continuous(breaks = round(seq(min(dat$y), max(dat$y), by = 0.5),1))