This replaces "12," "twelth," or "twelfth," with: "twelve:"
using System.Text.RegularExpressions;
string s = @"additional twelfth consecutive month(s) (""Renewal Term"") by giving";
Regex r = new Regex(@"\b12\b|\btwelth\b|\btwelfth\b");
s = r.Replace(s, "twelve");