From an answer given at Is there a limit in the number of GitHub organizations a single account can create?, it seems that
if you don't see a hard limit published, you'll have to consider the main guideline of "be reasonable".
Also some searching has found that at https://help.github.com/articles/what-is-my-disk-quota
For best performance, we recommend repositories be kept under 1GB each. This limit is easy to stay within if large files (typically, binaries) are kept out of the repository. If your repository exceeds 1GB, you might receive a polite email from support requesting that you reduce the size of the repository to bring it back down under 1GB.
Is my understanding correct that this is a per-repo limit? For example, can I have 2 0.5 GB repositories? Will a single .99 GB file affect this performance or 99 .1GB files?