How to validate that at least one checkbox should be selected?

The accepted answer abuses stuff to use in a way they are not meant to be. With reactive forms the best, easiest and probably right way is to use a FormGroup that holds your grouped checkboxes and create a validator to check if at least one(or more) checkbox is checked within that group.

To do so just create another FormGroup inside your existing FormGroup and attach a validator to it:

form = new FormGroup({
    // ...more form controls...
    myCheckboxGroup: new FormGroup({
      myCheckbox1: new FormControl(false),
      myCheckbox2: new FormControl(false),
      myCheckbox3: new FormControl(false),
    }, requireCheckboxesToBeCheckedValidator()),
    // ...more form controls...
  });

And here is the validator. I made it so you can even use it to check if at least X checkboxes are checked, e.g. requireCheckboxesToBeCheckedValidator(2):

import { FormGroup, ValidatorFn } from '@angular/forms';

export function requireCheckboxesToBeCheckedValidator(minRequired = 1): ValidatorFn {
  return function validate (formGroup: FormGroup) {
    let checked = 0;

    Object.keys(formGroup.controls).forEach(key => {
      const control = formGroup.controls[key];

      if (control.value === true) {
        checked ++;
      }
    });

    if (checked < minRequired) {
      return {
        requireCheckboxesToBeChecked: true,
      };
    }

    return null;
  };
}

In your template don't forget to add the directive 'formGroupName' to wrap your checkboxes. But don't worry, the compiler will remind you with an error-message if you forget. You can then check if the checkbox-group is valid the same way you do on FormControl's:

<ng-container [formGroup]="form">
   <!-- ...more form controls... -->

   <div class="form-group" formGroupName="myCheckboxGroup">
      <div class="custom-control custom-checkbox">
        <input type="checkbox" class="custom-control-input" formControlName="myCheckbox1" id="myCheckbox1">
        <label class="custom-control-label" for="myCheckbox1">Check</label>
      </div>

      <div class="custom-control custom-checkbox">
        <input type="checkbox" class="custom-control-input" formControlName="myCheckbox2" id="myCheckbox2">
        <label class="custom-control-label" for="myCheckbox2">At least</label>
      </div>

      <div class="custom-control custom-checkbox">
        <input type="checkbox" class="custom-control-input" formControlName="myCheckbox3" id="myCheckbox3">
        <label class="custom-control-label" for="myCheckbox3">One</label>
      </div>

      <div class="invalid-feedback" *ngIf="form.controls['myCheckboxGroup'].errors && form.controls['myCheckboxGroup'].errors.requireCheckboxesToBeChecked">At least one checkbox is required to check</div>
    </div>

    <!-- ...more form controls... -->
  </ng-container>

*This template is very static. Of course you could create it dynamically by using an additional array that holds the the form-data(key of FormControl, label, required, etc.) and create the template automatically by use of ngFor.

Please don't abuse hidden FormControl's like in the accepted answer. A FormControl is not meant to store data like id, label, help-text etc. and doesnt even have a name/key. All this, and much more, should be stored separate, e.g. by a regular array of objects. A FormControl only holds an input-value and provides all this cool state's and functions.

I created a working example you can play with: https://stackblitz.com/edit/angular-at-least-one-checkbox-checked

consider creating a FormGroup which contains your check-box group and bind the group's checked value to a hidden formcontrol with a required validator.

Assume that you have three check boxes

items = [
  {key: 'item1', text: 'value1'},      // checkbox1 (label: value1)
  {key: 'item2', text: 'value2'},      // checkbox2 (label: value2)
  {key: 'item3', text: 'value3'},      // checkbox3 (label: value3)
];

Step1: define FormArray for your check boxes

let checkboxGroup = new FormArray(this.items.map(item => new FormGroup({
  id: new FormControl(item.key),      // id of checkbox(only use its value and won't show in html)
  text: new FormControl(item.text),   // text of checkbox(show its value as checkbox's label)
  checkbox: new FormControl(false)    // checkbox itself
})));

*easy to show via ngFor

Step2: create a hidden required formControl to keep status of checkbox group

let hiddenControl = new FormControl(this.mapItems(checkboxGroup.value), Validators.required);
// update checkbox group's value to hidden formcontrol
checkboxGroup.valueChanges.subscribe((v) => {
  hiddenControl.setValue(this.mapItems(v));
});

we only care about hidden control's required validate status and won't show this hidden control in html.

Step3: create final form group contains below checkbox group and hidden formControl

this.form = new FormGroup({
  items: checkboxGroup,
  selectedItems: hiddenControl
});

Html Template:

<form [formGroup]="form">
  <div [formArrayName]="'items'" [class.invalid]="!form.controls.selectedItems.valid">
    <div *ngFor="let control of form.controls.items.controls; let i = index;" [formGroup]="control">
      <input type="checkbox" formControlName="checkbox" id="{{ control.controls.id.value }}">
      <label attr.for="{{ control.controls.id.value }}">{{ control.controls.text.value }}</label>
    </div>
  </div>
  <div [class.invalid]="!form.controls.selectedItems.valid" *ngIf="!form.controls.selectedItems.valid">
    checkbox group is required!
  </div>
  <hr>
  <pre>{{form.controls.selectedItems.value | json}}</pre>
</form>

refer this demo.