Because newsletter templates are available in all variety of lengths and sizes, there is really no limit to the content that you could potentially write. Keep in mind, however, that you should probably keep down your word count to a level that people are comfortable reading. While you may think everything that you are spouting off is fascinating, it's helpful to first sit down and make an outline of what you want your main message to be, or what the major news is that you are trying to share.
A trick to writing fresh content on a consistent basis month in and month out is to learn to look at your topic from all angles and all levels. This is beneficial for your readers, because you are creating a library of information that covers your topic inside and out. This is also beneficial SEO_wise, because you are systematically creating a collection of resources that covers your subject backwards and forwards. This creates a greater chance of your article turning up for searches on every aspect of your niche. Last but not least__learning to cover your topic from all angles and all levels of depth is beneficial for YOU, because it forces you to establish your expertise in your field. If you are not already an expert in your field when you start writing articles, you surely will be after a year of consistently writing articles on every conceivable angle of your topic each month.