Thanks for contacting Syncfusion support.
Please find the response for your queries below.
I am using xamarin forms and mvc, can I use 1 key for both or each needs it is own key?
No, you can’t get single key for both Xamarin and ASP.NET MVC. Each license key will represent as single platform. But, you can use both license key in your application by separating ‘;’. Also, in license generation itself separating license key with ‘;’ while generating with multiple platforms.
Whenever I upgrade to a newer nuget version, should I get a new key or key for the previous version should still work?
Syncfusion license keys are version specific. So, you have to generate the corresponding version license key when you upgrading to newer version of Syncfusion NuGet.