There are two places where you "claim" a number such as this.
The first is on your W-4 that you provide to your employer so that the employer knows how much income tax to withhold from your paycheck. On this form, claiming 1 means that you are asking the employer to base your income tax withholding on the IRS table that is adjusted for "1" withholding allowance.
The second is on your actual tax return (Form 1040, Form 1040A, or Form 1040-EZ). On this form, claiming "1" means that you are claiming the allowable personal exemption for yourself but not for anyone else (such as a spouse or a child).
If you need additional information in this regard, please let me know.
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