Actually read AppSettings in ConfigureServices phase in ASP.NET Core

That is the way you can get the typed settings from appSettings.json right in ConfigureServices method:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.Configure<MySettings>(Configuration.GetSection(nameof(MySettings)));
        services.AddSingleton(Configuration);

        // ...

        var settings = Configuration.GetSection(nameof(MySettings)).Get<MySettings>();
        int maxNumberOfSomething = settings.MaxNumberOfSomething;

        // ...
    }

    // ...
}