We do not know anything else--like your filing status, what state you are in, whether you have any dependents, whether you can be claimed as anyone else's dependent, etc, etc.....

 

The more allowances you put on your W-4, the less the employer will withhold for taxes.  

Try the IRS W-4 calculator and the taxcaster tool.

 

https://www.irs.gov/individuals/tax-withholding-estimator

 

https://turbotax.intuit.com/tax-tools/calculators/taxcaster/

Simple answer:  you will have $420 (4200 x 10%**) less withheld in a year ($8 per week).  But, you will most likely owe $420 at tax filing time. 

 

But taxes aren't simple.  It's best to use one of the calculators to get a more specific estimate for your situation. 

 

**Each additional allowance is  typically worth $4200 times your marginal tax rate.  But that's not a hard and fast rule.

I think the incremental income is falling in the 12% tax bracket rather than of the 10% bracket, so over the course of an entire year the reduction in withholding would be $504 instead of $420 (or about $9.69 per week instead of $8.07 per week).

I agree.  In his case some is at 12% and some at 10%.  So, somewhere between $420 and  $504.

If he owes less than $1000 at tax time, there will be no underpayment penalty.

There is no in-between in the withholding calculation unless pay varies substantially between pay periods.  At $23,000 per year received uniformly, the income per-pay period falls in the 12% bracket, so one withholding allowance equals $504 annually.

Basically it comes down to about $10 a week or less in difference ... will that really make it worthwhile now if this makes you owe it all back later ?