How can I get my visa number corrected before or during my 221?
There's no need for a correction. One does not apply to renew a visa, they just apply for a visa. Rest assured, the consular official will have access to your application and interview history as well as your travel history to the United States. If they have a question, they'll ask you at the interview. Also, one should not need to use an agent. Applying for a visa can be done online quickly, especially for just a B-1/2 visa. If an agent makes mistakes, the applicant is the one who bears the brunt of them and some of them are fatal to the application and possibly fatal to future applications. Lastly, agents cost money. Save your money and do it yourself.