Jak zliczyć wszystkie zaznaczone itemy z mat-select - Angular

<mat-select
            multiple
            formControlName="{{ items.name }}"
            [matTooltip]="displayValuesTooltip(items.name)"
            matTooltipPosition="above"
            (selectionChange)="onChange($event, items.name, form)"
          >
            <div class="filter-input-container">
              <app-filter-input></app-filter-input>
              {{items.name}}
            </div>
            <div class="custom-panel">
              <mat-option *ngFor="let item of items.content" [value]="item">{{
                item.name ? item.name : ( item.firstName && item.lastName ? item.firstName + ' ' + item.lastName : item)
              }}</mat-option>
            </div>