Hey Laura,
It might depend on exactly what you're trying to show in the report (i.e., is the focus just the number of people, the performance of the email or the performance of the leads). You probably want to create an email performance report with some smart list filters to restrict it to exactly the audience you're looking at. (See: Email Performance Report - Marketo Docs - Product Docs). That'll show you how many people received a specified email in a set time period, along with opens, clicks, etc.
You could also use a people performance report (People Performance Report - Marketo Docs - Product Docs ) to get the straight numbers - this report is really focused on looking at the growth of your database rather than the performance of an individual asset or program, but will allow you to set smart list filters to only show people who have received that email, and group them by key characteristics.
If your welcome email is part of a program with set success metrics, a program performance report (Program Performance Report - Marketo Docs - Product Docs) may also be useful, as it will show how many people are members of the program, and how many of them met your "success".
(I should note: you won't necessarily need all three of these, but one or other may be better suited to provide any additional information you're looking for in the report)
Hope that helps 
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