Rashi on Exodus 30:12/רש"י על שמות ל׳:י״ב

1כי תשא This has the meaning of obtaining: WHEN THOU TAKEST [THE SUM], (not of “lifting up” as in Genesis 40:13); as the Targum has it, תקבל. The sense is: when you wish to obtain the sum total of their number — to know how many they are — do not take their census by their polls but each of them shall give half a shekel, and you shall count these, and so ascertain their number.

2ולא יהיה בהם נגף THAT THERE BE NO CALAMITY AMONG THEM — for numbers (i. e. things that have been numbered) are subject to the influence of the “evil eye”, and therefore if you count them by their polls pestilence may befall them, as we find happened, in the days of David (II Samuel 24:10 and 15).