None. As long as you take 3 and they are not critical thinking you are fine. most do history, sociology, business, philosophy etc. If you can take law alevel thats fine, i found it helped hugely for my personal statement, work experience discussions, and understanding legal topics in the law entrance exams LNAT
Usually Universities don't have specific A- Level requirements to study Law. You will just need to meet the entry requirements. So, I would recommend doing A levels that you enjoy and think you will do well in. There may be a few A levels like general studies that universities will not accept. I would recommend taking a look at a few universities law pages for more info on this.