What does the minimal number of round of "nr" use Keccak to avoid “collision attack”? What does the minimal number of round of "nr" use Keccak to avoid collision attack?

$$nr = 12+2L$$
Cursory literature search suggests that the best collision attacks reach only six rounds of Keccak:


Kexin Qiao, Ling Song, Meicheng Liu, and Jian Guo, ‘New Collision Attacks on Round-Reduced Keccak’, in Jean-Sébastien Coron and Jesper Buus Nielsen, eds., Advances in Cryptology—EUROCRYPT 2017, April 2017, Springer LNCS 10212, pp. 217–243 (link, preprint)
Ling Song, Guohong Liao, and Jian Guo, ‘Non-full Sbox Linearization: Applications to Collision Attacks on Round-Reduced Keccak’, in Jonathan Katz and Hovav Shacham, eds., Advances in Cryptology—CRYPTO 2017, Part II, Springer LNCS 10402, August 2017, pp. 428–45 (link, preprint)


Of course, attacks only get better, so it would be imprudent to choose six rounds. 
 This state of affairs justified the use of a mere twelve rounds in KangarooTwelve at substantial speed increase over the twenty-four used by SHA-3.