What is the limit to the number of emails that can be in one particular stream within the Engagement Engine
I don't think there's anything but a theoretical limit. You can add as much content as you'd like, but does a 100 email nurture stream make sense for your business? 200? 1000?
That being said, I could envision dropping a ton of content into a stream so someone is never really exhausted. The reality is that only Marketo (or unnamed competitors) probably have that much content.
And much of that content would get deprecated after awhile, so the limits are irrelevant.
I believe the 15 stream and 100 engagement limit still applies for most instances.
You're correct. Those are the current limits for the number of streams within an Engagement Program and the number of active Engagement Programs per instance.