Is Quick Note free to download?

Yes, Quick Note is free to download from on the Mac app store.
In the free version you can create upto 3 notes. For more notes you can purchase the Pro version

What will I get after purchasing Quick Note Pro?

You can create unlimited notes after purchasing the Pro version. In free version you can create upto 3 notes.

Is Quick Note Pro one time purchase or subscription?

Yes. Quick Note Pro is one time purchase now and forever

I have two macbooks. Can I access the notes from one macbook to other?

Yes. You can do so by enabling iCloud sync in both the macs

How can I download the app on iPhone?

Quick Note is only available for mac os. It's not available on iPhone, iPad, Android and Windows.