Hi,
A simple Mac app to generate a sequence of numbers - no problem!
I develop apps for iOS and OS X, using Xcode and Swift.
You can see some examples of my work, and client reviews of my completed projects, on my Freelancer profile page.
It looks like you may be new to Freelancer. Before starting the project, there should be a clear understanding of the details, the deadline, and the milestone payments.
You don't say much about the details, or the user interface, so I'm taking you at your word, that it's a "simple" app!
If you'd like to reply to this message, we can talk about the details, before you make any decision on awarding the project.
Best Regards,
- Rob Dixon.