fixed "number" of plots using facet_wrap

Try this,

library(ggplot2)
library(plyr)
library(gridExtra)

dat <- data.frame(x=runif(150), y=runif(150), z=letters[1:15])

plotone = function(d) ggplot(d, aes(x, y)) + 
  geom_point() + 
  ggtitle(unique(d$z))

p = plyr::dlply(dat, "z", plotone)
g = gridExtra::marrangeGrob(grobs = p, nrow=3, ncol=3)
ggsave("multipage.pdf", g)