You can't directly compare mAh values, as they're an unit of electric charge. The amount of energy depends on voltage, too. Power bank manufacturers are basical

You can't directly compare mAh values, as they're an unit of electric charge. The amount of energy depends on voltage, too. Power bank manufacturers are basically running a marketing scam; the batteries they sell have 3.7V while USB uses 5V. What's more, the MBP battery voltage is something around 12V. So the 26,800 mAh power bank has a capacity of 26,800 mAh * 3.7V = 99,600 mWh = 99.6 Wh of energy. Meanwhile, the 15" MBP has a 76 Wh battery. So theoretically you can do about 1.3 charges; realistically I'd say one full charge.