For example, if I applied to Harvard for Chemistry and I got denied, from that point, are you unable to apply to Harvard forever, even if I was 100% accepted to other majors? Ok, can you apply to more than two majors so that if you get denied for one of them, you can still have a chance for the other majors and get into that school? Thanks.
As far as I know, it really depends on the school. Some have you apply to the undergrad colleges as a whole and in that case you wouldn’t be able to have two different chances of getting accepted for two different majors. Some you apply to a certain colleges within the university and in that case you could apply to different programs and get accepted into some but rejected into others. Usually it’s not specific majors, but different catagories of study that have their own college within the university. Commonly there will be a college of arts and sciences and a school of engineering that will require separate applications. I could be wrong but that’s my understanding of it.
Most colleges don’t accept by major, although if you have to apply to different schools within the college, the major you plan on needs to be within that school. You can take a gap year and apply again if you are turned down, but honestly that is rarely successful. Sometimes it is possible to transfer to a different school within the college, but you have to read the website info at each college to see what is involved, it is not always possible or easy.
Large public universities accept by college, not by major (ie., College of Communications, College of Business, College of Liberal Arts…) but you only apply once and may* indicate an alternate major. Not all universities even offer this possibility (in California especially). College of Engineering tends to be the hardest to get into. THis also applies to prestigious programs at universities with specialized colleges and some specific majors (like CS at UIUC or UWashington, or Annenberg at USC, or SFS at G’Town or SIS at American).
As for Harvard, they don’t admit by major. So it doesn’t matter. It’s just one application, where you indicate what fields would interest you. If you’re interested in sanskrit or Mythology AND you have evidence in your application (academic + EC) that it’s a true interest, it may help a bit, but otherwise, it’s not like UCAS where you make one application per “course” or field of study. In the US, it’s one application per university. If they distinguish by major (and often they don’t) you have to pick carefully. If you don’t know what to pick, choose “undecided”. Some universities (like Penn State, Ohio State, UCincinnati…) will let you choose a “branch” campus if you don’t get into the “main” campus.