* Geographic location: Literacy was far higher in certain areas (e.g., parts of the Byzantine Empire, some regions of China) than in others (e.g., most of sub-Saharan Africa, many parts of Europe).
* Social class: Literacy was overwhelmingly concentrated among the elite – government officials, religious leaders, merchants, and the wealthy. The vast majority of the population, regardless of location, was illiterate.
* Definition of literacy: What constitutes "literacy" is itself debatable. Could someone who could read but not write be considered literate? What level of reading comprehension is required?
Therefore, providing a single number for the 6th century's literacy rate is impossible and misleading. Any attempt would be a highly speculative, broad generalization. We only have fragmented evidence from which scholars can draw educated guesses, often region-specific and limited in scope.