You can inherit from a single parent downwards as far as you want...
A extends Object; B extends A; C extends B; etc
Thus, by implication, C extends B, A, and Object.
What you can't do is this:
E extends F,G
ie. extend from two parents at the same level. This is what Java refers to a multiple inheritance.
It's single inheritance. Each class inherits from 1 class only, but that class, in turn,inherits from one class, etc, all the way up to Object.
When your code refers to a method or variable in a class Java looks for that method/variable in the class definition. If it finds it, OK. If not, it looks in the definition of the superclass. If it finds it there, OK. If not, it looks in that classes' superclass etc, all the way to Object. I it doesn't find it in Object you get a compile error.