My net income is $40K in Maryland state. I am on F1 visa working on OPT . How much tax cut will I have every month?

If you did not complete 5 years on F1/OPT, you are exempt from residency and you do not pay SS tax and Medicare tax. You will pay federal and state income taxes.

Also note that, as a foreign national working in the U.S. you can claim only ONE or ZERO alowances on your W-4. this is done to ensure that enough taxes are withheld from your salary to cover your tax liability on the federal level.