Defence and defense are two spellings of the same noun. The spelling depends on the type of English.
In American English, “defense” is most common, so self-defense is correct.
In British English, “defence” is standard, so self-defence is correct.
In both cases, also make sure to hyphenate the term.
The same distinction applies to similar words, such as offence or offense, licence or license, and pretence or pretense. It’s important to choose one spelling and use it consistently. The QuillBot Grammar Checker can help you with this.