In Mandarin, there are some words called Measure Words those are very different from English. When you count something, you must put a Measure Word between the number and the thing you want to count.
We call these words Measure Word because when we count things, we measure its the shape or the size. For example, water doesn’t have a shape, but when you put water in a glass, we can say ‘It is a glass of water’. There are some Measure words in Mandarin. According to the shape or size of the things you count, you need to choose the right Measure word.
At the moment, we learned three Measure Words: 只(small animals)，个(most commonly used measure words. for now, it is for people)，口(when you talk about there are some people in my family).
Don’t forgot when the number is TWO, we don’t say 二, instead, we need to say 两（liang3）