If you put "0" then more will be withheld from your pay for taxes than if you put "1"--so that is correct. The more "allowances" you claim on your W-4 the more you get in your take-home pay. Just do not have so little withheld that you owe at tax time.
https://turbotax.intuit.com/tax-tools/calculators/w4/
But are you sure you will be a dependent on your parents' return? You are 21? Are you still a full-time student? If you are a full-time student then your parents can still claim you as a dependent.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
