trackByIndex(index: number, obj: any): any {
  return index;
}

<div *ngFor="let comment of comments; let i = index; trackBy: trackByIndex">
  {{ i, comment.text }}
  <div *ngFor="let answer of comment.answers; let j = index; trackBy: trackByIndex">{{ j, answer.text }}</div>
</div>

"comment 1": {
    "answer 1",
    "answer 2"
},
"comment 2": {
    "answer 1", // должен быть индекс 3
    "answer 2" // должен быть индекс 4
}

<div *ngFor="let comment of comments; let i = index; trackBy: trackByIndex">
  {{ i, comment.text }}
  <div *ngFor="let answer of comment.answers; let j = index; trackBy: trackByIndex">{{ answer.text }} {{counter++}}</div>
</div>

counter: number = 0;

import { Component } from "@angular/core";

@Component({
  selector: "app-root",
  template: `
    <ul class="nth-reset">
      <li *ngFor="let x of data; let i = index">
        Section {{i+1}}
        <ul>
          <li *ngFor="let y of x" class="nth-inc">
            {{y}} is item #<span class="nth"></span>
          </li>
        </ul>
      </li>
    </ul>
  `,
  styles: [`
    .nth-reset { counter-reset: nth 0 }
    .nth-inc {counter-increment: nth }
    .nth:after { content: counter(nth) }
  `]
})
export class AppComponent {
  data = [
    ["1-A", "1-B", "1-C"],
    ["2-A", "2-B"],
    ["3-A", "3-B", "3-C", "3-D"]
  ];
}