A handy utility for pluralising strings based on a numeric value, using standard English pluralisation rules by default.

StringPluralisation.cs

public static class StringPluralisation
{
	
	private static Regex ConsonantY = new Regex("([bcdfghj-np-tv-z]|qu)y$");
	private static Regex Sibilant = new Regex("(s|z|sh|ch)$");
	private enum PluralRule { Regular, Sibilant, ConsonantY }
 
	private static PluralRule GetPluralRule(string noun)
	{
		if (Sibilant.IsMatch(noun)) return PluralRule.Sibilant;
		if (ConsonantY.IsMatch(noun)) return PluralRule.ConsonantY;
		return PluralRule.Regular;
	}
		
	/// <summary>
	/// Pluralise a noun, or a phrase ending with a noun, based on the number of items it describes.
	/// </summary>
	/// <param name="number">The number of items the word describes.</param>
	/// <param name="singular">The singular form of the word.</param>
	/// <param name="plural">The text to use if there is more than one item. (If not supplied, a plural will be generated based
	/// on regular English pluralisation rules.)</param>
	/// <param name="zero">The text to use if there is are zero items. (If not supplied and an explicit plural form is provided,
	/// that will be used; otherwise, a plural will be generated based on regular English pluralisation rules.)</param>
	public static string Pluralise(int number, string singular, string plural = null, string zero = null)
	{
		if (string.IsNullOrEmpty(singular)) throw new ArgumentNullException("singular");
 		zero = zero ?? plural;
 
 		if (number == 1) return singular;
		if (number > 1 && plural != null) return plural;
		if (number == 0 && zero != null) return zero ?? plural;

		var rule = GetPluralRule(singular.ToLowerInvariant());
		var head = singular.Remove(singular.Length - 1);

		switch (rule)
		{
			case PluralRule.ConsonantY : return head + "ies";
			case PluralRule.Sibilant   : return singular + "es";
			default                    : return singular + "s";
		}
	}
}