A team has finished a game with 21-6 singles and extra-base hits 2 times since 1897, most recently San Francisco Giants on 8 June 1990. That is about one every 221,502 team-games.
Box score for the most recent one ↗
21-0 21-1 21-2 21-3 21-4 21-5 21-6 21-7 21-8 21-9 21-10 22-0 22-1
The whole distribution — Singles and Extra-Base Hits · All 18 shapes