Hi @aleccs
Here’s an example how to do it:
$my_number = get_field('mynumber');
echo do_shortcode('[add_to_cart_url id=' . $my_number . ']');
Please keep in mind that you need to put it inside The Loop.
If you have time, please learn more how the template works here: https://codex.wordpress.org/Theme_Development.
If you don’t have time to learn it, I suggest you hire a developer to help you out with it, and I’d recommend looking for one on https://studio.envato.com/ or https://www.upwork.com/.
I hope this helps 🙂
