From what I've been told, the reason for the abbreviation is that the number represents the number of characters between the two letters. The number is not pronounced. This is what initially confused the heck out of me - I knew what i18n was but didn't connect the dots. Only got raised when we were talking about Kubernetes being shorted to k8s :)
Yeah, even when submitting this Ask HN I had to change i18n to "i18n" because it got expanded after I posted it in.
Internationalization and accesibility, of course. What are some good resources on i18n (and l10n)?