After your order is completed and the SIM is shipped to you, your phone number is assigned.
After you receive your new SIM, you will be given your number at the end of the activation process.
If you require assistance with activation, read this article: I want to activate my Rocket Mobile SIM?
You can access your Rocket Mobile account by visiting the Rocket Mobile App or MyAccount at any point after activation is completed.
To retrieve your phone number, you have 2 ways:
In your Rocket Mobile MyAccount, go to "Home" by simply clicking on your name in the top right corner of the page. You'll then be routed to the "Personal Data" page
In your Rocket Mobile App, go to the menu (three lines) in the upper-right corner.
Then select "My Rocket" from the drop-down menu.
And then click on the "Account" tile.
You'll see the "Account" page, then you can see your phone number