CountAsync.cs

public static int Count<TEnumerable, TEnumerator, TSource>(this TEnumerable source)
    where TEnumerable : IValueEnumerable<TSource, TEnumerator>
    where TEnumerator : struct, IEnumerator<TSource>
{
    var counter = 0;
    using var enumerator = source.GetEnumerator();
    checked
    {
        while (enumerator.MoveNext())
            counter++;
    }
    return counter;
}

public static async ValueTask<int> CountAsync<TEnumerable, TEnumerator, TSource>(this TEnumerable source, CancellationToken cancellationToken = default)
    where TEnumerable : IAsyncValueEnumerable<TSource, TEnumerator>
    where TEnumerator : struct, IAsyncEnumerator<TSource>
{
    var counter = 0;
    var enumerator = source.GetAsyncEnumerator(cancellationToken);
    await using (enumerator.ConfigureAwait(false))
    {
        checked
        {
            while (await enumerator.MoveNextAsync().ConfigureAwait(false))
                counter++;
        }
    }
    return counter;
}