Charging multiple In Reach Minis

Hi, we run adventure challenges in the Amazon where participants carry a In Reach Mini 2 for safety and potential assistance.  Some of this Challenges are quite long and we need to charge the devices overnight from Generators or really bad power supplies in small settlements. 

  • How many Garmin Mini can we charge at the same time without risking of bricking the devices?
  • Could I use the set up below?
  • Could we double the amount of In Reaches charged from the set up below?
  • What is the most efficient/quickest way of charging over 20 of them?

I understand that they need a minimum 1A, but would appreciate some advice.

Thank you!