What is the symbol to refer to the set of whole numbers

Solution 1:

To summarize what has been said in the comments, there are no "official" symbols. Use whichever notation you feel most comfortable with, as long as it makes sense and can be easily understood by the general audience.

Some examples include:

$\mathbb{Z}_{\ge 0},\mathbb{Z}^{+}\cup\{0\},\mathbb{N}\cup\{0\},\mathbb{N}_0$

Also note that because of different conventions, what you refer to as "whole numbers" may or may not include zero. From Wikipedia:

There is no universal agreement about whether to include zero in the set of natural numbers: some define the natural numbers to be the positive integers {1, 2, 3, ...}, while for others the term designates the non-negative integers {0, 1, 2, 3, ...}.

Solution 2:

Except $\mathbb{N}\cup\{0\}$ we use $\mathbb{I}$ symbol too for $\{0,1,2,3,...\}$ and we call it the set of calculating numbers that it has only zero more than natural numbers. Also in some books it has been denoted by $\mathbb{Z}^{\geq0}$