How many cylinders can be exchanged at a time? </h1><p><style type="text/css" id="isPasted"><!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}-->2 empty cylinders can be exchanged at a time with our exchange box. We recommend owning a total of at least 3 canisters. That way, you can always have one in your machine, even as you're sending your other 2 empties in to be refilled.