The 50-image limit per gallery is a limit built-in to Juicebox-Lite. There is no such limit in Juicebox-Pro.
You should be able to create a gallery using the Lightroom plugin with as many images as you like.
All images will be listed in the 'config.xml' file.
However, if you were to use Juicebox-Lite rather than Juicebox-Pro, then only the first 50 images in your gallery would be displayed.
If you see an error such as "the original ORF file cannot be found" then this message is generated by Lightroom itself (rather than the plugin).
Please double-check the location of your source file (and that the image's filename has not changed since you added it to your project) as this may be the cause of your problem.