He is saying, the current design is 4 LED'S in series per string at 1 amp
Then ...
4 strings = 4 amps
8 strings = 8 amps
16 strings = 16 amps
The design can be changed from 4 LED's in series per string, to 3 LED's in series per string, to allow more overhead voltage.
But then 33% more "3 LED" strings are needed to keep the same total number of LED's in the overall design.
4 LED's per string x 2.9 v per LED = 11.6 Volts per string
3 LED's per string x 2.9 v per LED = 8.7 Volts per string
Yes, you can parallel Constant Current Sources to provide the total amps needed for the exact number of string attached = 4, 8 or 16 strings.
The LM317 is very small and cheap ( $0.50 each ) CC supply - use one per 4 LED series string?
It can supply 1 amp given a small ( Vin - Vout ) voltage difference = keep watts dissipated low and attach to metal.
LM317 is a Linear supply = not very efficient unless controlled / constant small Voltage Differential
The problem with a Car Battery is ... the voltage can vary from 12 Volts to 14.4 volts !
Use a CV Buck-Boost Inverter ( at rated amps ) in front of the array of LM317's ?
If your battery is only 10.5 Volts either ...
a) the car engine is cranking or
b) the battery is dead
In either case, your LED's should not be ON.