We used to carry only 2 when it was windows xp based tablet PCs (MIL spec ones). Which was good enough for the last 15 to 20 years or so of paperless operation. Now that we're in our second gen iPad ops we get a spare iPad on each aircraft (additional to the personal ones for each pilot) instead of the spare windows tablet we carry at the moment. Saves 4 kg of weight and quite a few issues with updating and power supply, not to mention training. Dunno if the eQRH is even available for windows XP at all.