There is no hard limit to the number of people or organizations that can be added to a pipeline, but performance may degrade after 500 prospects are in a single pipeline. It is not common for a GP to actively network with that many prospects of a single kind at once. Performance on the pipeline page is still being improved, so loading times should continue to improve. If you want to send a newsletter to hundreds of contacts, it is currently best to use an external CRM. Better support for newsletters is on the roadmap, but there is no set date for it.