Create Calculation field with Get(RecordNumber) then under storage options click "Do not store - recalculate as needed. Then insert this calculation as a merge field onto your layout.
Thanks S Chamblee.
How about if I want to store that number for reference in anonther report?
Take a look at Replace Field Contents and its serial number feature. But be careful with this one, if several people are accessing the database all at once, they'll all see the numbering done by each other and that can be possible Chaos for your data.