Hi - is there a reason that Marketo would say my audience is"0" when I am using the same Smart list as a previous email? This has been happening when I am trying to set up emails for the past few days.
There may be the case that the people may have been updated since then (i.e., since the last program run with the non-zero audience) making them ineligible for the SL criteria. Did you try creating a separate stand alone smart list with exact same criteria and check if you see any people there or not?
Then it's likely that people who qualified earlier may have been updated and/or deleted. You spot check the people from the previous program run to see if they do actually have all the attributes/activities for qualifying for the smart list filter or not.