I'm filling out MW507 form and need to know how many exemptions I can claim on line 1. I'm a single father with a 4yr old that I have full custody off and makes $50,000.

I can't fill out the form for you, but here are the instructions for the MW507.  It looks like you would have 1 exemption for yourself, 1 for your child, and 1 if you file as head of household.  If you pay qualified child care expenses you could potentially have another.  The form along with the instructions are included in this Link:

http://www.mva.maryland.gov/human-resources/Career-Service-or-Executive-Service/all-w4-instructions....