Oof, yeah, like others have said, there's not a simple way to determine that retroactively. The only thing I can think of for recording the data moving forward is maintaining a spreadsheet with a log of the SmartList member values outside of Marketo. This could be something you just check daily and input, or maybe create an email subscription to send a daily report on the # of members on the list each day. And then maybe if you have a developer write a script and macro to easily extract the necessary tally from the daily reports into a single spreadsheet, you could automate it. But it's not pretty, either way.
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