I don't think Marketo currently supports reporting on Smart Campaign Step out of the box so any tracking/reporting you'll do would have to be manually set up.
There are a couple ways to do this:
You can see the total number of leads waiting, overall. You should be able to do this by going to the Campaign Activity Report, selecting the campaign(s) desired, and then running the report. I thought there was a way to watch which Wait Step they are in. I'm not sure since I don't have one running now.
http://community.marketo.com/MarketoArticle?id=kA050000000KypOCAS
You will also see how many people are in various Wait Steps (I think) on the Campaign Summary tab of the actual campaign.
As Charlie said, you could also use the Email Performance Report (also on the campaign summary tab) to figure how the current email stats and where people are in the flow.
A better way may be to convert this to an Engagement Program where Marketo will handle most of that for you. Once it is setup, use smart campaigns to pull people out of the old system and into the new one...or at least stop accepting new leads into the old one.