The following is an excerpt from Introduction to the Mathematics of Finance by Roman:
As a more concrete example, suppose that IBM is selling for $\$100$ per share at this moment. A $3$ month call option on IBM with strike price $\$102$ is a contract between the buyer and the seller of the option that says that the buyer may (but is not required to) purchase $\color{blue}{\bf 100}$ shares of IBM from the seller for $\$102$ per share at any time during the next $3$ months.
In this example is the number $\color{blue}{100}$ in blue arbitrarily picked by the author? Is it relevant to the concept of "call option"?