A team has finished a game with 21-3 singles and extra-base hits 7 times since 1897, most recently Seattle Mariners on 22 September 2004. That is about one every 63,286 team-games.
Box score for the most recent one ↗
20-8 20-9 20-10 21-0 21-1 21-2 21-3 21-4 21-5 21-6 21-7 21-8 21-9
The whole distribution — Singles and Extra-Base Hits · All 18 shapes