user role

You need to add some custom PHP code to the right place, responsible for showing quantity. May be it allows to use filter to change the output or it could be required to edit original code directly:

if (current_user_can('special_user_role_id')) {
  // code to show pallete
} else {
  // original code to show quantity
}

Do you know in which file will it go in woocommerce

No. It is more related to the WooCommerce forum or just google it – How to customize woocommerce product quantity output?

I need something like this if customer 1 logged in shows pallet else show quantity