Do I need to file a separate form for my mom and one for my dad, or I only need to file one form for both mom and dad together?

You must petition each of them. There are no dependents on immediate relative visa petitions. They must also apply for a visa at the National Visa Center for an additional cost and with additional forms that may cause their disqualification. I strongly recommend an appointment with a competent and experienced immigration attorney before there are any unexpected complications. Good luck.

Generally, a separate application needs to be filed for each beneficiary. Significantly more information is needed, however, in order to accurately answer your question and to provide a legal analysis of the steps to be taken for your parents to gain immigration legal benefits. This includes, for example, information about your own immigration status (Are you a U.S. citizen? Are you a Permanent Resident? Do you have some other status?); information about your parents (How and when did they enter the U.S.? What is their current immigration status? What immigration-related applications every before have been filed by or for them?); Etc. There really is no substitute for you and your parents to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.