[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
    class ContentItemCountValidationAttribute : ValidationAttribute
    {
        public ContentItemCountValidationAttribute(int numberAllowed)
        {
            this.numberAllowed = numberAllowed;
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            //CODE FOR RESOLVING DISPLAY NAME
            // ------------------------------------
            var propertyDefinitionRepository = EPiServer.ServiceLocation.ServiceLocator.Current.GetInstance<IPropertyDefinitionRepository>();

            var propertyName = validationContext.MemberName;
            var propertyDefinitionId = ((ContentData)validationContext.ObjectInstance).Property[propertyName].PropertyDefinitionID;
            var propertyDefinition = propertyDefinitionRepository.Load(propertyDefinitionId);
            propertyDefinition.LocalizationService = LocalizationService.Current;
            var displayName = propertyDefinition.TranslateDisplayName();
            // ------------------------------------

            var contentArea = (ContentArea)value;
            if (contentArea == null || contentArea.Count < numberAllowed)
            {
                return new ValidationResult(
                    string.Format("{0} should have at least {1} content(s), please provide content(s).", validationContext.DisplayName, numberAllowed),
                    new[] { validationContext.MemberName });
            }
            return ValidationResult.Success;
        }

        readonly int numberAllowed;
    }

You should be able to cast it to ContentData to get validation of number of content area items to work on properties for blocks as well. 

Thank you for quick help on this.