You can either apply an ngIf on the element using the index:
<div *ngFor=" let tweet of singleCategory; let i=index">
<div *ngIf="i<2">
{{tweet}}
</div>
</div>
If you don't want the wrapping div, check out template syntax:
<ng-template ngFor let-tweet [ngForOf]="singleCategory" let-i="index">
<div [ngIf]="i<2">
{{tweet}}
</div>
</ng-template>
Preferably you first/instead filter the elements in your component using filter to prevent unnecessary loops when displaying your data:
public get singleCategory() {
return this.categories.filter((item, index) => index > 2 )
}
There is also the option of creating a pipe. (See the linked duplicate)