It may be different for some schools, but some allow up to two majors and four minors.
There are some students who take a double major. I have never been made aware of any institution that would allow more than that.
The simple answer is number of credits. Generally each department within a college or university sets forth X number of credits (and classes) are required for a major and Y number of credits (and...
You may take as many as you want and can afford. There is no limit other than the number offered and your own financial restraints.
You must declare and complete at least one major, but yes, you can complete a minor as well.