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> 「どうよ衛宮、桜の出番○×回に対して僕三回だぜ？ すげー、片手でも指が余るって正直調整おかしくない？つーかぁ、兄より優れた妹など存在しねー」

I think it's reffering to the number of scenes he starred in.  
I googled it and found this:

> 片手で/に余る=件数と指の数。 片手に余る＝5より多い。 片手で余る＝5未満。
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2016-05-08T09:48:15.390
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2016-05-08T16:18:49.843
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question
words
片手でも指が余る meaning?
> [片手]{かたて}でも[指]{ゆび}が[余]{あま}る

Picture a toddler counting something by folding his fingers one by one. He is
also too young to know to use the same finger more than once.

Now, how many 指 (fingers) does a 片手 (one hand) have? The answer is ' **
_five_** ' as the thumb is also considered a 指 in the Japanese-speaking world.

余る means "to be in surplus". If your fingers are in surplus after the
counting, there should be less than five of the items, agreed? So, there
should be 1, 2, 3 or 4 in total quantity. If there are none, you would not
count in the first place, and if there were 5 or more, you would not have any
fingers left unfolded in the hand being used, correct?

The speaker says 「[僕三回]{ぼくさんかい}だぜ」 = "As for me, (only) three times." His
little sister Sakura has appeared OX times (and I must assume OX means "many"
here because the speaker is clearly unhappy about making fewer appearances
than his sister does).

Thus, 「片手でも指が余る」 means the quantity/frequency is less than 5.
CC BY-SA 3.0
2016-05-08T16:18:49.843
34020
2016-05-08T16:18:49.843
34011
answer