(It is estimated that )150( of the 300 )languages( that were once spoken by American Indians) exist.
An estimated 相当于It is estimated that...的句型意思是：据估计
其他小括号里的都是languages的修饰词,后半句150后面省略了个languages
完整的句子是an estimated 150 languages exist.也就是
It is estimated that 150 languages exist.