Don't worry and ask me questions if you don't get something.
I'll start by saying this. mAh doesn't tell you the total capacity of a battery unless you know the voltage of the battery.
Meaning a 5000 mAh battery with 7.6 V will have 2x capacity than 5000 mAh battery with 3.8 V.
When you convert the capacity in Wh then Voltage of battery don't matter anymore and you can compare it directly.
mAh x V/1000 = Wh
A 26800 mAh battery pack has 8 cells inside (each is 3350 mAh, 3.6 V). Thus the total capacity is 26800 mAh x 3.6V/1000 = 96.48 Wh
iPhone XS has a 3174 mAh, 3.8 V battery = 3174 mAh x 3.8V/1000 = 12.06 Wh
Now you have the battery capacity of both, battery pack and iPhone. Just divide these to get total number of charges (assuming 100% efficiency). Actual number of charges will be less.