It's going to be easier to look at the stack state in-depth for this one.

After Player A Soul's Fire, Kalamax and Swarm Intelligence trigger. We'll assume Player A orders them Kalamax, then Swarm Intelligence. There are currently no copies of Soul's Fire on the stack (only the triggers that will eventually make the copies) and so, only targets for the original spell have been chosen. Stack state: SF, K, SI

After Swarm Intelligence's trigger resolves, 2 copies of Soul's Fire are put onto the stack and Player A chooses targets for them. If Player B casts removal at this point, 3 "Soul's Fire" spells will have an invalid target and do nothing. Stack state: SF, K, SF, SF

Next, the 2 copies created by Swarm Intelligence will resolve. This happens before Kalamax's trigger because they were added to the stack after. Stack state: SF, K

Next, Kalamax's trigger resolves, creating 2 more copies of Soul's Fire and Player A picks targets at this point. If Player B chooses to use removal at this point, the remaining 3 Soul's Fire spells will do nothing, but 2 have already resolved. Stack State: SF, SF, SF

Effectively, Player A gets to re-choose 2 targets. Player A announced their intentions for targets of spells that hadn't been put on the stack yet. Player B decided to take an action before some of those spells got put onto the stack, so Player A is allowed to change decisions for targets of spells put on the stack after Player B's action.