The macs battery is about 50 or so watt hours, your average $40 external battery is around 70. Disregarding inefficiencies of charging a battery from a battery, about one.
The largest Mi power bank is 16000+ mAh, smallest is 5000. MPB is 6000, phone is about 3500. So you could get about 1 or 2 charges depending on battery size of course.
I don't know the specs of the new MBPs, but my mid-2015 MBP's battery is 7530 mAh at full charge. You can get portable charger batteries that are much more than that these days; this one [1] is 26,800 mAh, which in theory could charge my MBP 3.5 times. In practice, less than that, but still probably at least two full charges.
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.
A Macbook Pro (or indeed any laptop) battery operates at a voltage much higher than that of your phone or a powerbank. The actual power available from the battery (in Wh) can therefore only be compared with another battery's quoted capacity if the voltage is taken into account.
A MBP battery might be a 11.1V battery, while a portable power bank capacity is almost certainly quoted as a summation of the capacities of the 3.7V cells inside. The capacity of the power bank must therefore be divided by 3 to give the equivalent capacity in mAh at 11.1V.
That is kind of cool -- but how many batteries is it going to take, in practice, to juice up a MBP from empty?