Are ghostwriters usually the ones who write the first few chapters of a novel?

No. First most fiction writers do not use ghostwriters. Secondly, if a ghostwriter is hired, they would normally write the whole novel. In the few cases where they don’t write the whole novel, it is because the named author started the book but was unable to finish it. For those books, the ghostwriter would pick up where the named writer left off, usually after the first few chapters (although even then, the ghost would probably edit/rewrite the beginning too.)