Is Google plex actually a number?

No, but a "googolplex" is. It is equal to 10googol, a googol being 10100.

In other Words, a googolplex is equal to 101000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.

Writing that number out in it's full form is impossible, as there is not enough storage space in the world for it. In fact, if you were to attempt to write out the number, there would not be enough space in the universe to write it.

It is however a number, despite the impossibility of truly conceiving it's size.