I'm assuming you have a content area in your Container Block where you can add additional card blocks ? 

If this is the case can you implement IValidate e.g. 

    /// <summary>
    /// Validates a <see cref="T:Netcel.Models.Blocks.VideoSlideBlock"/> instance
    /// </summary>
    public class VideoSlideBlockValidator : IValidate<VideoSlideBlock>
    {
        private List<ValidationError> _errors;
        private VideoSlideBlock _instance;

        /// <summary>
        /// Validates the <see cref="T:Netcel.Models.Blocks.VideoSlideBlock"/> instance
        /// </summary>
        IEnumerable<ValidationError> IValidate<VideoSlideBlock>.Validate(VideoSlideBlock instance)
        {
            _instance = instance;
            _errors = new List<ValidationError>();

            ValidateVideoSourcesContentArea();

            return _errors;
        }

        /// <summary>
        /// Checks that the VideoSourcesContentArea contains at least one VideoFile
        /// </summary>
        private void ValidateVideoSourcesContentArea()
        {
            if (_instance.VideoSourcesContentArea != null && _instance.VideoSourcesContentArea.Items != null)
            {
                var videoFound = false;
                foreach (var item in _instance.VideoSourcesContentArea.Items)
                {
                    var contentLoader = ServiceLocator.Current.GetInstance<IContentLoader>();

                    VideoFile file;
                    if (contentLoader.TryGet(item.ContentLink, out file))
                    {
                        videoFound = true;
                        break;
                    }
                }
                if (!videoFound)
                {
                    _errors.Add(GetMissingVideoError("VideoSourcesContentArea"));
                }
            }
            else
            {
                _errors.Add(GetMissingVideoError("VideoSourcesContentArea"));
            }
        }

        /// <summary>
        /// Returns a missing video error
        /// </summary>
        private ValidationError GetMissingVideoError(string propertyName)
        {
            return new ValidationError
            {
                ErrorMessage = propertyName + " must contain at least 1 video item",
                PropertyName = propertyName,
                Severity = ValidationErrorSeverity.Error,
                ValidationType = ValidationErrorType.AttributeMatched
            };
        }
    }

Assuming you are talking about the content area, in addition to what Minesh suggested, you can also create a custom validation attribute that can allow you to set the range.

Something like below (untested):

   // Custom attribute validator
    [AttributeUsage(AttributeTargets.Property)]
    public class ContentAreaItemsAllowedAttribute : ValidationAttribute
    {
        private int _max;
        private int _min;

        /// Constructor with given max
        public MaxItemsAttribute(int min, int max)
        {
            _min = min;
            _max = max;
        }

        // Check if it is valid
        public override bool IsValid(object value)
        {
            string error = $"The number of items in the Content area should be between {_min} and {_max} items!";

            var contentArea = value as ContentArea;
            var count = contentArea?.FilteredItems?.Count()
            if (count > _max || count < _min)
            {
                ErrorMessage = error;
                return false;
            }
            return true;
        }

        /// Return Result
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var result = base.IsValid(value, validationContext);
            if (!string.IsNullOrWhiteSpace(result?.ErrorMessage))
            {
                result.ErrorMessage = $"{validationContext.DisplayName} {ErrorMessage}";
            }
            return result;
        }
    }
}

And then in your Block/Page:

[Display(
    Name = "Video Items",
    Description = "Video Items.",
    GroupName = "Content",
    Order = 10)]
 [ContentAreaItemsAllowed(1,5)]
 public virtual ContentArea VideoItems { get; set; }