Hey @mmntm,
There’s unfortunately no default way to go about doing this—i.e., no way of doing this from within the plugin admin directly and such.
You would have to get clever and use some custom coding at this time. We can only provide very, very limited support for custom coding (read more here), but I wrote up a snippet for you that does the things you listed above.
Check out that snippet here → https://git.io/vViMM
Copy and paste the contents of that snippet into your theme’s functions.php file and you should be good to go! The quantity will be set to 1 and will not be changeable by the user.
Now, if you want to further customize the appearance of that field from here, you would have to take the reins on that—but I hope this helps get the quantity-limiting functions you describe!
Sincerely,
George