6.
Picture this, to help understand it.
Pretend person A already had the 6 in their wallet. They took out the $6 loan anyways (and now have 12). They then spend the $6 they had to start with, then hand back the 6 back to the bank that they borrowed.
The only money spent was the original 6. Taking money, and handing it back, didn't "spend" any more (assuming no interest or transaction fees).
Person A borrowed 6, and spent it, and now needs to pay it back. But that paying it back isn't spending.
So they spent 6, they don't have to "spend" another 6 paying back person B. They do have to earn $6 to pay it back, but they never had 6 in the first place, they went into debt and obligated their future self to earn that 6. But they never had to earn $12 (which they would if they spent $12), they just had to earn $6 (the loan wasn't "earning" the money), cause they only spent 6.