If you can understand this, you have a pretty good understanding of Rx, Async, Generics, and Extension Methods.

BatchAsync.cs

public static IObservable<TResult> BatchAsync<T, TResponse, TResult>(
    this IObservable<T> source,
    int count,
    Func<IEnumerable<T>, CancellationToken, Task<TResponse>> process,
    Func<TResponse, IEnumerable<TResult>> resultSelector
    )
{
    return source
        .Buffer(count)
        .SelectMany(batch =>
            Observable
                .FromAsync((token) => process(batch, token))
                .SelectMany(x => resultSelector(x))
        );
}

Usage.cs

Observable.Empty<ThingToProcess>()
    .BatchAsync(100, (batch, token) => apiCapableOf100PerRequest.ProcessAsync(batch, token), (response) => response.Things)
    .BatchAsync(25, (batch, token) => apiCapableOf25PerRequest.ProcessAsync(batch, token), (response) => response.Things)
    .Subscribe(thing => {
        Console.WriteLine("Thing was processed: {0}", thing.Id);
    });