How Many Vials Of Humalog Do I Require To Control Diabetes For 42 Days?

I take 3 doses of Humalog before meals, 8 units each. I use the 3ml vial which apparently contains 300 units of insulin. I am travelling out of the country for 42 days and calculated that I need 5 vials for the entire trip. however, when I take a dose of 8 units, the vial says I have used 30 units each time. I do not want to run out while I am away. What am I doing wrong? And if I use 24-30 units a day, how many vials do I reguire for 42 days?
Much thanks.