Would a monthly newsletter be one program? Or one program for each month?

If you are just trying to see open rates, and which links people are clicking on, it may not matter.  Now, you could set up different success paramters for each newsletter, and if that is the case, I would seperate it out and have a different program for each one.  In fact, if you did it as an email program type, it would be really easy for you to do different A/B tests.  That is what I would suggest.  

If you do a program each month, then you can run an email performance report to show all emails for a time period and look at how your rates compare with benchmarks.

Josh is correct.

If you keep one program for multiple emails, you'll never really see how each newsletter affects the lead performance over time and you'll have somewhat meaningless counts in the Program Summary.

We have a program for each of the 5 different publication newsletters that we have.  I started out doing a program for each month or even yearly.  It got cluttered easily.  So I just have 5 programs by publication name, and set up a foldering system within the programs by year and then by month.  I track the link performance (topic or headline clicks) and email performance report monthly (opens).  Topic of interest, I cross reference with Google Analytics, Marketo email newsletter is the referrer, and I track the time spent on the page.  

Thanks everyone. Since this is for an internal mailing list, I think I will keep it simple and just set up one prgrams and do as Michelle suggested: just have folders by year and month.