We have no exact number. Only generalisations. According to Harris in the book "Ancient Literacy" (1989) the population in the Roman world could have had as high as a 30% literacy rate.

The issue when discussing literacy is that you're often dealing with the educated elite or (in Rome in particular) educated slaves who were literate almost purely for administrative purposes. It's important to note that literacy did not equal social success (as in the instance of slaves. Though one could argue life as a literate slave doing administrative work was probably preferable to life as a slave working latifundia).