what is the max charge current of BQ2057?
The available charge current is determined by the capability of the outboard transistor and the CC output current.

The applications section of the datasheet has typical charge circuits such as the one below:



There is a guide on how to choose that transistor starting on page 15 of the datasheet which shows how to calculate the pass element power dissipation which is the key factor in making the appropriate  choice.

Note that the maximum output current from the CC pin is limited by power dissipation to 40mA.