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.
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.