IncrementalList.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Threading.Tasks;
using Windows.Foundation;
using Windows.UI.Core;
using Windows.UI.Xaml.Data;

namespace UniversalTestApp
{
    public class TestClass
    {
        public async void Test()
        {
            IncrementalList<int> list = new IncrementalList<int>(async (count, offset) =>
            {
                await Task.Delay(1000); //Simulate delay
                //Replace this stuff with proper database queries
                List<int> results = new List<int>();
                for (int i = offset; i < offset + count; i++)
                {
                    results.Add(i);
                }
                return results;
            });
        }
    }

    public class IncrementalList<T> : ObservableCollection<T>, ISupportIncrementalLoading
    {
        private Func<int, int, Task<List<T>>> _loadMoreAction;

        public IncrementalList(IList<T> items, Func<int, int, Task<List<T>>> loadMoreAction) : this(loadMoreAction)
        {
            foreach (var item in items)
                Items.Add(item);
        }
        
        public IncrementalList(Func<int, int, Task<List<T>>> loadMoreAction)
        {
            if (loadMoreAction == null)
                throw new ArgumentNullException(nameof(loadMoreAction));
            _loadMoreAction = loadMoreAction;
        }

        IAsyncOperation<LoadMoreItemsResult> ISupportIncrementalLoading.LoadMoreItemsAsync(uint count)
        {
            return ProcessMoreAsync(count).AsAsyncOperation();
        }

        private async Task<LoadMoreItemsResult> ProcessMoreAsync(uint count)
        {
            var items = await _loadMoreAction((int)count, Count).ConfigureAwait(false);
            var results = items?.Count ?? 0;
            HasMoreItems = results >= count;
            if (results > 0)
            {
                // Ensure we update the list on the UI thread
                await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
                    () =>
                    {
                        int oldCount = this.Count;
                        foreach (var item in items)
                            base.Items.Add(item);

                        base.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, changedItems: items, startingIndex: oldCount));
                        base.OnPropertyChanged(new System.ComponentModel.PropertyChangedEventArgs(nameof(Count)));
                        base.OnPropertyChanged(new System.ComponentModel.PropertyChangedEventArgs("Items[]"));
                    });
            }
            return new LoadMoreItemsResult() { Count = (uint)results };
        }

        public bool HasMoreItems { get; private set; } = true;
    }
}

Another thing I just noticed: You never use the count and offset values that are passed into the delegate. Your query method PoblarListaProtocolos needs to take these parameters. Ie the two parameters will tell you "get the next 50 items, starting at offset 100"