Tmp.txt
Experimentally, I get the same results as you (on a Linux machine running kernel 3.16 which is otherwise mostly idle): with seq 1 12773, cat tmp.txt picks up the output of wc; with seq 1 12774, cat tmp.txt picks up an empty file. So why is there a difference between 12773 and 12774, but the results are pretty reliable below that value?
tmp.txt
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