The teen numbers (11–19) and 20 look different from single-digit numerals because they use two digits. Children need explicit instruction to recognize these written forms.
You already know the numerals 0 through 10 — what each one looks like and what amount it stands for. Numbers from 11 to 20 are the next step, and they look different because they are written with *two digits* side by side. This is something new: instead of a single symbol like "7", you see two symbols together like "1" and "3" making 13. The left digit and the right digit each play their own role in making the number.
The numbers 11 through 19 are called teen numbers. Most of them have "teen" in their spoken name — thirteen, fourteen, fifteen, and so on — which can help you remember they belong to this group. But 11 and 12 are a bit tricky because they are called "eleven" and "twelve," not "oneteen" or "twoteen." Even so, they are still written with a 1 on the left and a 1 or 2 on the right: 11 and 12. The left digit is always 1 for all the teen numbers, because there is one group of ten inside each of them.
Learning to recognize these numerals means knowing what the symbol looks like *and* what quantity it represents. When you see 17, you know it is more than 10 and less than 20 — and more than you can show on your fingers alone. 20 is special: it is the first number where the right digit is 0. The 2 on the left means two groups of ten, and the 0 means no extra ones. Practicing by matching the written numeral to a picture of that many objects (like 14 apples or 18 stars) builds the connection between the symbol and the amount. Over time, seeing "16" will feel as immediate as seeing "6" — your brain will recognize the whole numeral at once, not just letter by letter.