Convert everything to watt-hours and calculate...
Example.
8,500mAh 1s = 8.5 amps x 3.7v (nominal voltage) = 31.45Wh
Say you want to charge that five times and each time you charge it, you are starting at 50% capacity or state of charge (ie, you are using 50% of the battery out on the track)
8.5 amps x .5 (50% capacity) = 4.25 amps x 3.7v = 15.725Wh x 5 charge cycles = 78.625Wh
You'll want to add a little bit for inefficiencies in the charger, so let's say 78.625WH x 1.10 = 86.5 Wh You'll need a 6s battery with a minimum of 86.5Wh to charge your 8,500 mAh 1s battery five times from a 50% state of charge.
Therefore you need...
6s battery nominal voltage is 6 x 3.7v = 22.2v
86.5 Wh/22.2V = 3.89 Ah or 3,890mAh
In this scenario, a 4,000 mAh 6s battery is what you'd need.