how many pitcher vs. batter at bats are significant?

[sam h]

The quality of the individual batters and pitchers, and their righty/lefty splits, are obviously more important. So for practical purposes this doesn't matter much - ie, it shouldn't be a decisive factor very often in making pinch-hitting or bullpen decisions.

However, that doesn't mean that you can't tell anything from small samples of data. It doesn't have to be "statistically signficant" for you to draw an inference. It's just not that useful except maybe in very extreme cases.