A Bluk trade is when you are sending multiple instances of the same card in a trade. In a bulk trade you will have two numbers that appear: 

  1.  Number of owned card instances: A number in the top corner of the card you are trading / receivng this shows the number of owned card instances by you or the trading partner 
  2.  Number of card instances being traded: This number can be found on the right side of the screen between two arrows that show adding or removing card instances. You can also select to add the max amnt meaning all total card instances of the card you own.