Greencards - how many greencards do I need?
You don't need many greencards per se because a green card is, actually, a license to stay in America and travel around. You only need one. Greencards are personal (they have your personal detalis printed on them) and non-transferable (meaning that you cant give up your greencard so that someone else gets it; you can't sell it, give it away, auction it on eBay or anything like that). Greencards are given out by the Department of State and people who win them and pass the rest of visa process should consider themselves very lucky considering the competition they went through.
Remember, applying for multiple greencards does not increase your chances to get a green card; instead, submitting multiple greencards applications will disqualify all of them and you will lose your chance for a green card this year - without even knowing it, as you will not be notified that your greencards applications are disqualified. Even worse, it might happen so that the next year your country of nativity will not be eligible for diversity visa immigration program, so you might effectively lose a chance to win a greencard for a number of years if you apply for multiple greencards.
If anyone is trying to pitch you into buying greencards or getting fake papers, never even try to do it. You are looking at permanent disqualification from green card program for greencards fraud or, even worse, criminal prosecution.
You only need to apply once a year.