in chinese, number is always followed by quantifier when there's noun after them：number＋quantifier＋noun（三＋个＋兄弟） 个is commonly used after number and before noun. so: 我有五个兄弟。

hola friend, when you want to say amount of something you have to add quantifier before noun in Chinese. 当你想说一些事物的数量，你应该在名词前加上数量词。 so you'd better say 我有五个兄弟，it will be best。个is a quantifier.