Did you know that there was a difference between a child's being able to count up to twenty and that child being able to count twenty things accurately? The first is called Rote counting. This is reciting the names of numbers in order. The other is called one to one correspondence. This is an important part of the base for learning to do addition and subtraction.
A common sign that a child may not have a good grasp of one to one correspondence is their consistently skipping a number (usually one in the teens) like sixteen each time he or she counts. Often a child is able to count items correctly up to ten or twelve but starts miscounting somewhere in the teens. There is an easy way to check on this. Line up ten or twelve items like blocks or macaroni pieces and have the child count them, touching each one as it is counted. Do NOT count with them. If that is done correctly, then add a few more pieces so that there are seventeen or eighteen items and ask your child to count them.
If there is a problem, then start working on counting things around the house a daily activity you do with your child. This helps them develop a sense in their minds of how numbers are related to each other. This is called the internal number line. It is usually helpful to start with amounts of items that are one or two numbers more or less than the amount your child can count accurately. Having a well developed internal number line is very important to learning beginning arithmetic. Building a solid foundation now will help your child succeed in the future!!