You can keep a count of number of times you want to generate this pattern in a variable. (If this value is increasing everytime button is clicked, global variable can be used for keeping the count ... say maxCount) and then you can loop it as below:
for (int i = 0; i < maxCount; i++)
{
message.AppendLine();
if (i==maxCount-1)
message.AppendLine("Plot " + i+1);
else
message.AppendLine("Plant " + i+1);
message.Append(string.Format("$:{0} ", Cash), Color.Gray, TextMarkup.FontDecrease);
message.Append(string.Format("F:{0} ", Food), Color.Gray, TextMarkup.FontDecrease);
message.Append(string.Format("C:{0} ", Concrete), Color.Gray, TextMarkup.FontDecrease);
message.AppendLine(string.Format("M:{0}", Metal), Color.Gray, TextMarkup.FontDecrease);
message.AppendLine(string.Format("Time: {0}", Time), Color.Gray, TextMarkup.FontDecrease);
message.AppendLine(MessageBuilder.Elements.CreateLink("btnBuild", "Build", ""));
}