How many lines of code do professional programmers write per hour?

it depend on the problem ,the logic  and experience of developer.

  that's all

I think the number of lines added is highly dependent upon the state of the project, the rate of adding to a new project will be much higher than the rate of a starting project.

The work is different between the two - at a large project you usually spend most of the time figuring the relationships between the parts, and only a small amount to actually changing/adding. whereas in a new project - you mostly write... until it's big enough and the rate decreases.

it depend what are they codding, if they have a static code like css or html they can code more than100 lines in a hour, but if they have a logic code like javascript, php functions they writte less lines.