No, this is not functionality built into data assist.
You would need to create a record set that queries the orders table to see how many tickets for an event have been sold, use the SUM function on the quantity column and filter the query by the event ID column:
SELECT SUM(quantity) AS NumberSold FROM OrderDetails WHERE orderEventID = 1
this will return the number of registrations sold.
you can then create an if statement around the add to cart button so it will only display if the number sold is less than the number you have:
<?php if($row_Recordsetname['NumberSold'] <= 250) { ?>
<Add to Cart Code>
<?php } ?>
