It depends how you are charging them.
If through the USB-C port on the battery then the battery charging circuit will decide the charging rate depending on the voltage, if using a PD capable charger, and max current available.
If you are charging through the battery terminals then 1C or less is normally considered correct where C = The capacity of the battery in mAh e.g for 1750mAh battery charge at 1.75A or less.