You can easily find the number of project followers at any time by viewing the Follow Project widget from your project page.
The last line of the widget shows the number of community members following your project. There may be a difference in the number seen on this front end widget and the number of recipients in the newsletter. The reason is that when a user clicks to subscribe to the Follow project widget, the number is instantly increased by 1 in the front end of the project and an email verification link is sent to the user to confirm their email, only once the user verifies their email, will the number increase in the Newsletter section.
So to find the exact number of current recipients it is best to create a draft newsletter and view the recipient list as detailed in the article How to send newsletters to project followers. You can also determine their screen names and email.