To calculate the total number of pages, you would need to make an additional query to the database (see function implementaion). From a performance perspective, adding an extra database query every time you use pagination might not be ideal.

They are only doing one query.
Implementation:

/// Get the total number of items and pages
    pub async fn num_items_and_pages(&self) -> Result<ItemsAndPagesNumber, DbErr> {
        let number_of_items = self.num_items().await?;
        let number_of_pages = self.compute_pages_number(number_of_items);

        Ok(ItemsAndPagesNumber {
            number_of_items,
            number_of_pages,
        })
    }

    /// Compute the number of pages for the current page
    fn compute_pages_number(&self, num_items: u64) -> u64 {
        (num_items / self.page_size) + (num_items % self.page_size > 0) as u64
    }

This method returns BOTH number of pages and items with just one sql query, that why there is this helper method.
There is no perfomance perspective to worry about.

For ref:
Implementation of the num_pages which is currently used:

   /// Get the total number of pages
    pub async fn num_pages(&self) -> Result<u64, DbErr> {
        let num_items = self.num_items().await?;
        let num_pages = self.compute_pages_number(num_items);
        Ok(num_pages)
    }

It uses num_items which is also used by the other function.

And I want to expose the total number of items not the total number of pages because thats already being exposed.
See:

Let’s start from the end:

this is the current response

{
    "results": [...],
    "pagination": {
        "page": 1,
        "page_size": 25,
        "total_pages": 1
    }
}

Would you like to include total_items in the response?

by calling num_items_and_pages instead of num_pages? and propagate them into the JSON response?

Yes exactly!

This would give us more Information with no extra sql queries

Sure, that sounds good. go for it 🥇