What is the origin of Tal's expression "They can only take one at a time"? The expression refers to aggressive attacking style where one side leaves multiple pieces attacked, but the opponent cannot take full advantage of it since "they can only take one at a time". Was this said by Tal in an interview or in a book (assuming he was actually the one who said it)? Did he say it originally in Russian? If so - what is the exact Russian expression he used?
I have seen this referred to/attributed to a couple of Tal's games. It is quite likely that he used the saying more than once. If you study enough of Tal's games, you'll see that he developed a technique of not retracting his (minor) pieces when they were en prise. 

The first reference that I was able to find was in his Havana-1966 game against Bjorn Brinck-Claussen. (Quite likely this position, where he's just moved Ra8!

[Title "Tal v Brinck-Claussen, Havana 1966 "]
[fen "R2r2k1/6pp/1p3pn1/3b2q1/3P4/P4QP1/BP3P2/6K1 w - - 0 1"]


The saying is also often  attributed to his 1968 game against Chikovani, where Tal plays 19 Bxe6 instead of retracting his dark-squared Bishop.

[Title "Tal v Chikovani, 1968"]
[fen "1qrr2k1/p3bp2/bpn1Bnpp/6B1/3P4/P1N2N2/1PQ2PPP/3RR1K1 w - - 0 1"]


As you can see, in both cases Tal leaves multiple pieces hanging, a psychological strategy that he's used to his advantage.

Hope that helps.