How do you introduce a number pattern?
Start with smaller numbers - up to 10 - and build up. Talk about how each ten frame has "1 more" counter than the last. Show how 11 and 1 more makes 12, 12 and 1 more makes 13, 13 and 1 more makes 14 and so on. Explain that this - 11, 12, 13, 14, 15 - is a number pattern.