If you have up to 100 people on your site at any one time, you need 100 numbers. And it's best to have a couple more in reserve. If you run out of numbers, call tracking will start repeating them, and you won't know which of two users shown the same number actually called.
Combined call tracking
This option combines static and dynamic call tracking. Static is used where displaying a separate number to each user is not possible (offline advertising) or is simply not required. Dynamic is used where maximum information is needed (online advertising).

This combined method can even be used across different types of online advertising. For example, displaying a single phone number to all users who come from organic search results is beneficial because search engines don't reveal the search query that brought the user to the site. However, in contextual advertising, where you pay for each click on a keyword, it's crucial to use dynamic call tracking.