In your deal properties, you should have a property called Date entered “Proposal Sent”, which records the date when a deal moved into that stage.
If you just want to see how many proposals were sent during a certain period, you can build a custom report filtering deals where that date falls within your timeframe. You can then group the results by deal owner to see how many each salesperson sent.
Just keep in mind that this property only shows the most recent date the deal entered the stage. So if someone moves a deal back and then forwards again, it will overwrite the previous date.
Hope this helps!