What is the taxable amount working on a H2B visa earning $500 a week

H2-B is a work visa. It is subject to taxes just like the U.S. workers.
If you spend less than 183 days, there may be some benefit as per tax treaty between your country and the U.S.

Okay, $500 per week is about $26,000 per year.

Assuming you are single and CAN claim the $5,450 standard deduction plus your $3,500 personal exemption, your TAXABLE income would be $17,050.

The tax on that will be about $2,150.