In a typical 3-game MLB Wild Card Series, only about 14 to 17 players are used in high-leverage situations, while the remaining 9 to 12 players either do not play at all or only enter during low-leverage blowouts.
In the longer Division Series (DS) and Championship Series (CS), the high leverage core expands slightly to 16 to 19 players, while the number of completed unused players drops significantly.
Most of the additional usage is on the pitching side, with more starters and relievers needed to get through the longer series.
More bench players might be deployed in favorable matchups.