The most water you can use is 15-ounces. You'll have to measure how much liquid goes into your travel mug. That said, I don't recommend using that much water for a single k-cup. I've found that the best tasted for a single k-cup comes in at 8-ounces. You can use two though to do it, but measure how much water goes in your travel mug, divide that in half and use that many ounces for each k-cup. So, for example, let's say you have a 16-oz mug. Then use 8 ounces with one k-cup then another 8 with the second k-cup.
Hope this helps.