Function to check if limit is 0 in my theme file

Hey, I need a function to run in my theme functions.php to know if limit has been reached or not?

This is because I have replaced the add to cart form with a download form which I added through my theme. This download button allows members to download file and create order straight away without having to add it to the cart.

So i need your function to check if limit is 0.

If ($limit == 0){
not showing the download button
}