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There is no limit for number of allowances you may claim of W4 form - http://www.irs.gov/pub/irs-pdf/fw4.pdf - each allowance is approximately equal to $3600 deduction (for 2009)
You may write EXEMPT on the line 7 - "I claim exemption from withholding for 2008, and I certify that I meet both of the following conditions for exemption.
You can also use the Withholding Calculator on the IRS website at http://www.irs.gov/individuals/article/0,,id=96196,00.html for help in determining how many withholding allowances to claim on your Form W-4 to give you correct income tax withheld.