Async methods cannot use ref or out parameters. You can however use this Ref wrapper which is valid in terms of syntax

Ref.cs

 public class Ref<T>
    {
        public Ref() { }
        public Ref(T value) { Value = value; }
        public T Value { get; set; }
        public override string ToString()
        {
            T value = Value;
            return value == null ? "" : value.ToString();
        }
        public static implicit operator T(Ref<T> r) { return r.Value; }
        public static implicit operator Ref<T>(T value) { return new Ref<T>(value); }
    }
    
// usage
public async IAsyncEnumerable<JObject> GetAllCounted(Ref<int> count)
{
  await foreach(var item in GetAll())
  {
      count.Value++;
      yield return item;
  }
}