there's no "minimum" amount, because it matters about the level of your achivement. you could be getting distinction for 100 certificates, but it wont' be as good as 3 "high distinction/gold medal" certificates. if your performance will be average, i would say do around 3-4 in a year, so considering its half of the current year and a full last year that they ask in the application - do around 5-6 comps/extracurriculars. but then again its subjective, there are people who had minimal certificates and still got in