Working With SME-Slides

How many slides are needed? The minimum number is needed to present the content that will meet the objectives. 

I couldn't provide any guestimate number, though. There are too many unknowns, including how much text would be on each slide, as well as how you're measuring seat time.  

When presenting live, slides should ideally have relevant graphics and images with minimal text that supports what the speaker is saying. (Unfortunately, real life isn't ideal, and slides often show exactly what the speaker is saying...) 

That ideal setup would apply online if there's a narrator. But, if not, then all the text does have to be on the screen at some point. 

However, it good to remember that slides are free (well, unless someone is paying per slide). So instead of cramming a slide full of text, that same text can be spread across multiple slides. In other words, use one point per slide instead of all points on one slide.

Or related points could be revealed via an interaction.

In my experience, those decisions aren't made by the SME. Figuring out interactions and such is up to the instructional designer.