How do I determine the maximum current for charging a li-ion battery?
If you are charging the battery through the phone then this will have the charge controller circuitry between the 5V charge supply and the battery.  You CANNOT/MUST NOT just connect a battery pack to a power supply and expect it to charge without fire and or explosion.  

The charge controller in the phone will limit the current supplied to the battery pack to be within the limits specified by the battery manufacturer to ensure that the battery is not damaged.  Supplying the phone from a 5V source that has a higher current capability will not make the battery charge any faster.  If it did then you would run the risk of damaging the connector on the phone or even melting the tracks on the PCB within the phone.  Small USB connectors that I have used have a contact rating of up to 1A on the power lines.

If you want to build a charging station to charge multiple phones at a time then you need to have a power source that can supply up to the maximum charge current taken by the phone down each of the charging leads.  These can all be in parallel but I would place a diode in each of the positive supply lines to prevent the possibility of any current flowing from the battery back to the charger.