Sure, pull that data sample and run some analysis.
Marketo Support might be able to. You could also verify the # of API calls to get that data. It's probably one per lead on an extraction.
There's no count-only endpoint, unfortunately. Recommend cursoring over all the data (and discarding it, makes things easier!) for a week or so to get a baseline.
Alright thanks guys. I'll check with support first to see if it's available.