Answering your last question first, inductors in parallel are calculated similar to resistors; two in parallel will have half the inductance of a single one, three in parallel will have 1/3, etc.
There is no “current limit” for the LT8390, as the current limit is set by an external resistor, and the high current portion of the circuit will pass through external FETs rather than internally. However, multi-phase topologies are typically a much better way to go for really high output current designs, but they are more complex, and can only be accomplished with IC’s designed for such.
Since you are only using this as a buck, those are the equations you use.
“LSP” refers to inductor (L) sense resistor positive input (assumes current passing from Vin to Vout).
For calculating IL, using 30% for delta IL% is a good value. The Iout(max) they refer to on page19 is the max inductor current rather than load current. The average current should be the ratio of output voltage to the input voltage times the load current. Then, from that and using 30% for delta IL%, you can calculate delta IL. From this you can calculate Iout(inductor max), and from that, you can determine Lbuck minimum for inductor value…
As I stated above, the biggest limiting factor (other than being able to lay out a board which is capable of handling this current) is the inductor. All of the current required to feed the load must pass through this device, and few are spec’d to handle anywhere near what you need.
I must say that what you are trying to accomplish is very difficult. It sounds like you have little to no experience designing switching regulator circuits before. If so, doing an 8.1V, 100A charging circuit is a little like diving off a 30 meter platform and trying to stick the landing. If you have become an expert at diving off the 1 meter, 3 meter, and 10 meter levels, you have a much better chance, but even then, things could easily go very wrong.
However, if doing this as a novice, just as you would likely reach the water when you jump off a 30 meter platform, there’s a pretty good chance you will reach 100A (briefly), but the results are likely to be catastrophic. There are so many places for things to go wrong. I would strongly suggest you start with something a lot less demanding before working your way up to something like this.