Where I live, it's a surname, spelt Jacobi and pronounced ja KOH bee in English and yah KOOuh bee in Afrikaans. I associate it with rather down-market popular/country music. I've never seen it used as a fn;
Jacob is rare among English-speakers and Jakobus or
Jacobus is widely used by Afrikaans-speakers. Out of all of them, I prefer Jakobus but would never use it unless I'd married into an Afrikaans family.