Once again this comes down to simple mathematics.
Powerbank: 10000mAh (full at 90%)
Phone Battery: 1500mAh
Calculation: (10000mAh x 90% x 80%) / 1500mAh = up to 5 times
(Why 90%? Assuming the power bank is well maintained in good working condition and can conserve up to 90% power)
(Why 80%? An estimate of 20% power is consumed during the charging/ discharging process)