Add constraint for a maximum number of a ticket per order

src/pretix/base/models/items.py

@@ -111,6 +111,8 @@ class Item(LoggedModel):
     :type hide_without_voucher: bool
     :param allow_cancel: If set to ``False``, an order with this product can not be canceled by the user.
     :type allow_cancel: bool
+    :param max_per_order: Maximum number of times this item can be in an order. None for unlimited.
+    :type max_per_order: int
     """
 
     event = models.ForeignKey(
@@ -203,6 +205,13 @@ class Item(LoggedModel):
                     'canceled by the user until the order is paid for. Users cannot cancel paid orders on their own '
                     'and you can cancel orders at all times, regardless of this setting')
     )
+    max_per_order = models.IntegerField(
+        verbose_name=_('Maximum amount per order'),
+        null=True, blank=True,
+        help_text=_('This product can only be bought at most this many times within one order. If you keep the field '
+                    'empty or set it to 0, there is no special limit for this product. The limit for the maximum '
+                    'number of items in the whole order applies regardless.')
+    )
 
     class Meta:
         verbose_name = _("Product")