Everyones situation is different but I'll give you my years of thought on this matter...
You can fly with one battery but it's not really practical. If you only wanted to take a few photos and either have planned it all out or don't really care what you get, one battery might work. I don't recommend having just one battery if you can afford a second. I'd recommend a second battery over any other purchase. Two batteries is a good number to have. If a person is on a budget, two batteries is enough. It's basically a second chance to get a good photo or video and it allows you to fly in two different areas without having access to a charger and waiting an hour. So two batteries is enough or doable. I fine that three batteries tend to be the sweet spot. I seldom find myself using more than two unless I'm getting several spots for flying. I'd say I use two batteries 70% of the time and three batteries 20% of the time. I think three is the best bang for your buck. I'd also recommend trying three batteries only only buy more once you know three won't work for you. Yes, having more than three can't be bad and is only better but I'd say in most cases most people are loosing value at that point. I'd say having four or even five is going to just cost too much money for the return you get from their use.
For those who have 5, 6, 7, 10 batteries... good for you. I'm sure you need them and put them to good use. You are probably also the few that need that many. My advise is for most people.
I've love to have 20 or 30 batteries, that would be great. But I'd never use them all at one time (it would only serve to make them all last longer) so I would not get my moneys worth.
So:
1= save your money and get another
2= doable and would address 70% of most people's use (not 70% of fliers)
3= sweet spot
4 or 5 = might be slight over-kill but you know you are never going to want for a charged battery
6+ = only needed by a very few people or people who have no issue paying more for batteries than the Mavic. Professional use.