Thanks, I found the small print. The minimum number of flight credits needs to be 6:

This bonus applies to eligible Flight Pass activities completed between January 1 and December 31. Eligible Flight Pass activites are defined as the purchase of an eligible Flight Pass of 6, 8, 10, 16 or 20 flight credits (excluding Unlimited Flight Passes and Flight Passes for Business); the usage of every 10 eligible flight credits in conjunction with a Flight Pass for Business; or the duration of the travel period when you purchase an eligible Unlimited Flight Pass, on a monthly basis. Following an eligible activity, it may take up to three weeks for eUpgrade Credits to be deposited into your aircanada.com eUpgrade account. eUpgrade Credits are valid through February 28, 2019.