You could navigate to Analytics --> Lead(Person) Performance report. And use the drill down feature to identify which campaign the leads have originated from. You can also find more details about it here - People Performance Report - Marketo Docs - Product Documentation
Hi Rita Job,
Like Sonali mentioned, The people performance report would be the best way to track the number of new leads that were created in Marketo and the drill down feature to further dig into the source.
Also in case you want to specify a specific date range that you want to narrow your report down to, you can either do that by clicking on the "Person Created at" highlighted in the below snapshot,
Or your can also specify it in the smart list section of the report as shown below and then use the drill down feature.
This can help you narrow down your date range in the people performance report.