Counting to twenty extends children's range and introduces them to the teen numbers, which are the first instance of two-digit numbers. This is typically challenging because the names don't follow a consistent pattern.
Use a number line visible in the classroom. Point to each number as you count. Use song and rhythm to help remember the sequence. Practice the teen numbers (11-19) with extra emphasis.
You already know how to count from one to ten — you can say the words in order and match each word to one object. Counting to twenty adds ten more numbers, but these new numbers have a special challenge: their names don't follow the same pattern that comes later (like twenty-one, twenty-two, …). The numbers eleven through nineteen are called the teen numbers, and each one has its own name that you just have to learn.
After ten comes eleven, then twelve — these two names are the trickiest because they sound the most different from their meaning. Then come the "teen" words: thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen. You might notice that "thirteen" sounds a little like "three," "fourteen" sounds like "four," and so on. That hidden pattern can help you remember the order. Finally, after nineteen comes twenty.
When you count objects to twenty, you do exactly what you did when counting to ten: say one number word for each object, in order, touching or pointing as you go. You should only say "twenty" once — after you have pointed to the twentieth object. If you run out of objects before you reach twenty, or if you say a number without touching an object, the count won't be right. This "one number, one touch" rule is called one-to-one correspondence, and it is the most important part of counting correctly at any number.
Counting to twenty is also your first look at what happens after ten — the idea that numbers keep going past ten, then past twenty, then much further. Each time you practice the sequence, you are building the mental number line that will help you with bigger numbers later. The more you practice — counting steps as you walk, counting snacks, counting objects in pictures — the more automatic the sequence becomes, so you can focus on what the numbers mean instead of trying to remember their order.