Complex sentence: A public exists for every issue being discussed at a particular time, so that there are as many publics as there are issues, each public coming into being when its issue is first raised and going out of being when the issue is resolved.
Split into simpler sentences: A public exists for every issue being discussed at a particular time. Thus there are as many publics, each coming into being when an issue is first raised and going out of being when the issue is resolved.