Kakuzu's capabilities with Raiton: Gian are far underrated by many members on NB currently. The potential of this B-ranked jutsu was equal to that of Kakashi using TWO Raikiri's, why would Kakashi need to use 2 Raikiri's against a B-ranked Raiton Jutsu? Personally I believe rank placement on all of Kakuzu's techniques are incorrect being "B-ranked" anyhow this is getting off-topic.
Here Kakashi had to use two Raikiri's (S-rank) to counter Kakuzu's Raiton: Gian; a B-rank technique.
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Kakashi then praises Kakuzu on how skilled he is in general, but referred heavily to how he used his previously displayed Raiton technique: Gian:
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I want you guys to pay attention to the boxed in words: High Caliber, Perfectly, and Lightning. All fitting words, in which will furthermore prove that Kakuzu's Raiton: Gian ability shouldn't be underestimated like it is now. Kakashi had admitted that Kakuzu mastered, and performed a high caliber Raiton-based technique (Raiton: Gian), he later in his praising sentence goes on about stating that in order to have performed such a high caliber jutsu Kakuzu must have mastered Raiton perfectly.
So if Kakuzu had mastered his Raiton nature perfectly, and was able to execute Raiton: Gian to be able to multiply itself to attack multiple enemies present (Shikamaru, and Choji) why exactly could he not execute more than two? I believe Kakuzu could emit a great amount of Raiton: Gian against a single, and/or multiple opponent(s). Take a look at this Databook Page:
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Not only does the picture above support the idea of Raiton: Gian multiplying into many spear-like currents, but it basically blatantly hints to the public that it is perfectly capable of doing such a feat. Pay attention to the underlined words: Multiple, and Convergence.
I'll be using the mathematical definition for convergence due to that fact that I'm talking about multiplying Raiton: Gian.
Now yes, it is correct that Raiton: Gian has a finite (limit) however, it isn't measurable due to the dependency of the amount of opponents Kakuzu must be facing in order to determine the limit of Raiton: Gian. Kakuzu only multiplied Raiton: Gian by two because Shikamaru, and Choji were the only targets, and Ino was behind a tree so she wasn't in his line of sight.