UWP is for univeral apps like Windows 10 devices (Laptop, Tablet, Desktop, Phone, Xbox (soon)) etc. think "metro"/Win 8/8.1/10 store apps. WPF is the Windows Presentation Foundation which utilizes Xaml, an XML/HTML-esque language for designing user interfaces. WPF is more for the desktop and utilizes .NET for compatability between Vista, 7, 8, 8.1, 10, etc. UWP uses Xaml for its apps as well. For example, to create a hamburger menu in Xaml - via UWP - I do the following:
Code:
<RelativePanel>
<Button Name="PopoutPanelButton"
RelativePanel.AlignLeftWithPanel="True"
FontFamily="Segoe MDL2 Assets"
FontSize="20"
Height="45"
Width="45"
Content="" Click="PopoutPanelButtonClick"/>
</RelativePanel>
<SplitPanel>
... code omitted
</SplitPanel>
Now in WPF that may be different as WPF is more for desktop applications - like ones that are portable between computers. UWP have to either be downloaded from the store or sideloaded into the system. WPF can be zipped up or an installer created for users to install the app.
What's the benefit of UWP vs WPF? UWP is nice to put on multiple device platforms
running Windows 10. WPF is nice to create cross-compatible applications for various systems that don't utilize the "Windows Apps"/"Metro" style apps.
To answer your question, both will handle 50 buttons.