Charging is simple maths

To charge a 5S 5000mah sized pack at 1c (5 amps) requires a charger with at least 105 watts output

21v x 5 amps = 105 watts

At 2c

21v x 10 amps = 210 watts


Your power supply will generally be rated 20% higher than your charger

100 watt charger - 120 watt power supply
250 watt charger - 300 watt power supply


Every charger is slightly different in the voltage and current requirements needed to achieve a certain max output

Most high end chargers (above 150 watts or so) need a supply voltage higher than 12v (15 - 30v)

You dont need a multiport charger to charge more than one pack at once

A simple parallel cable / charging board will allow any charger to charge them at the same time

Cheers