Capacity of a single publish instance

I don't believe there is any existing standards that have been articulated in the way you want. The capacity of a publish server is very dependent on the application deployed on it. So the maximum number of pages a publish server that makes heavy use of the query API will be different than for one that use the query API minimally as one example. The number of variations on this theme are pretty much limitless which is why typically you calculate capacity using a formula - http://docs.adobe.com/docs/en/aem/6-0/manage/hardware-sizing-guidelines.html#Publish environment specific calculations. 

As a general rule the amount of content in your repository (templates, pages, or digital assets) should in theory be unlimited as long as your structure the content properly and you have enough disk storage. However there may be practical limitations that will vary based on your application design - a poorly design application or a poorly designed content taxonomy can cause performance problems, however a general rule I wouldn't consider the amount of content in capacity planning beyond ensuring you have enough physical storage.