Short answer.Long answer:
It's really hard to tell. A strong opponent for one group can be steamrolled by another group, and vice versa.
Two examples from Cradle of Darkness.
1) A gigantic Jaguar statue. It had supernatural everything, and at one point it swallowed a 10 shift hit like someone had thrown a ball of yarn. However, the attack skill was fairly low, so all in all, it didn't do much damage to the group, and they were able to wear it down with, I think, 1 consequence for a total of 6 players. My plan was to have a tough fight that would give them a bit of trouble and force them to take a few consequences. Didn't really work out that way.
2) As a ramp up to the final conflict, I'd thrown a dozen vampires at the group. They were fairly low powered, but not to be underestimated. In any way, I'd thought this would be a lot like swatting flies, but it took a really long time until they were actually down, far far longer than I'd expected.
One of the great things about Fate is the option for a concession. Once you learn how to embrace that, there's no need to worry about the right amount of opponents (see short answer). If you've got too many or too strong opponents, let the players concede or deal with what happens when they are taken out (which doesn't have to be death). If they get stomped into the ground the first couple of conflicts, that's fine, you can always have them come at the problem from another angle and have them take a detour. And if you set the opposition too weak, so be it. The characters caught a lucky break this time around.
Another question would be, what does "balanced" even mean? The big bad doesn't send exactly the right amount of minions to deal with the heroes, he sends what he sends. Maybe he's short on staff, maybe he's prone to overkill, maybe he just wants to delay the characters, and so on. There are a lot of valuables when it comes to opponents, and it doesn't always have to be "balanced".
It's more important in any conflict, to have an idea about what you are fighting about. The goals the two opposing sides are pursuing. If you focus on those goals, you can continue the story either way, no matter which side wins, the story continues. Try to find a way to make even losing interesting.
For example: You players want to hit a group of minions for the location of the big bads hiding place. If they win, they know where to go, if they lose, they don't. Which is fairly anticlimactic. So instead, you can set the goalposts a bit different. The players will get to the fortress no matter if they win or lose. But if they win, they get the location and can go themselves, if they lose, they will be captured and thrown into the dungeon, where they need to free themselves before they can take on the big bad.
If you look at it a bit more from that angle, the exact number of opponents doesn't matter all that much, as you will always find a way around, and while doing so, you will learn what different configurations of opponents can do to your group.