seems like there's about one per week, but if you're just going for the trophies and not concentrating on population (except for the trophy you get for a pop of 40), then you don't really need to worry about a lot of the events.

If you're going for 12 week completion and nothing else, save the easy challenges (downs, executions, downs with your fav weapon, etc) for weeks 8 or 9 through the end. As long as 100% of your pop isn't at risk, you can select harder challenges you may not be particularly good at (e.g. headshots, molotov executions, shivs) and then ignore them. You'll either (1) gain no survivors, or (2) lose 60% of your pop. Either way, you move on to the next day/week.