I just want to know how many quizzes we are allowed to submit at once. I heard that some people are allowed to submit two quizzes at once. Can anyone help please?

Some people who have a proven track record of good quizzes that require minimal editing can submit more than one at a time. It's a pretty small and select group.

You really should try to work on only one quiz at a time. You usually learn things in the editing process that should be applied to future quizzes.