Depending on the HDD vendor/model. We had hot spares and cold spares. On one build, we had a bad batch of drives. We built the array on a Friday, and left it for burn-in running over the weekend. On Monday, we came in to a bunch of alarms and >50% failure rate. At least they died during the burn-in so no data loss, but it was an extreme example. That was across multiple 16-bay rack mount chassis. It was an infamous case though, we were not alone.
More typically, you'd have a drive die much less frequently, but it was something you absolutely had to be prepared for. With RAID-6 and a hot spare, you could be okay with a single drive failure. Theoretically, you could lose two, but it would be a very nervy day getting the array to rebuild without issue.