The unit number is, in simplest terms, the basis upon which all other numbers are defined.
In the real numbers (and all number systems contained within the reals), this unit is 1.
All integer numbers are merely multiples of 1. A series of amending, or including (adding), the value of one to a preexisting value produces the next whole value greater. Consider a multitude of "1"s (or tallies) represented symbolically with the other numbers in our numbering system. The concept of a number is merely a representation of a quantity of units.
The imaginary numbers are indicative of the same continuum that the real numbers are. The imaginary numbers are merely represented as quantities, or multitudes, of the imaginary unit. The imaginary unit is i.