Yes stacks are unlimited, but it's not as good as it sounds. The passive may look good, but let me explain something to you. Draven is based on the first 15 minutes of playing. You have to play super aggressive and go for those early kills, and make sure you're denying creeps. If you're up against any other ADC and you have equal CS and kills, that means you're behind as Draven because of how hard it is for him to position in teamfights. This passive is a negative feedback loop; if you don't get ahead you basically have no passive since you lose so many stacks on death.