Plotting using the .NET ObjectARX wrappers is a bit different. The basic method you need to use is to create a PlotInfo object (which will contain your layouts to plot), validate the PlotInfo object, and then go through the plotting procedure by calling the appropriate methods in PlotEngine. The number of copies, for example, is set in the BeginDocument method I believe. For additional help, refer to the ObjectARX developers guide documentation under "Plot API".
Hope this helps,
- Scott