Most graduate schools request three letters of recommendation, but some fellowships request four. If you’re applying to a PhD program, many students can be admitted with one well-informed letter from someone you have done research with, and the others can provide additional context.
Unless they are all very strong, your application is unlikely to be helped by additional letters beyond the number requested. So asking more professors to write letters for you may not solve your problem, if there is one.
Some professors may tell you that they don’t have enough information to write a strong letter. If you sense there is a concern, you should think about it. Are you asking the wrong people to write letters? Is there something in your academic record your professors are justifiably concerned about? Are the programs you plan to apply to an appropriate match to your accomplishments and abilities? Instead of requesting more letters, try to understand and address their possible concerns.