At level 20 you have 4 attacks, but then you can action surge twice, so 12 attacks + bonus action + they get a free rapid strike since apparently they're samurai + strength beyond death gives them one more action so another 4
TOTAL: 4+4+4+1+1+4 = 18 Attacks
So it's the right number but the wrong way getting there lol. Was this a nerdsnipe?