A big hot shoe speedlite is about 60-70ws equivalent max. So with 200ws you will have getting on for two stops more light, and they recyle in about one second which is a quarter of the time most speedlites take at full power. They are impossibly slow to use for portraits on full power, even with fresh batteries. And of course you get a modelling light with studio flash.
As for how many you need, well basically I think that less is more, but this is how it works for me.
There should only ever be one main light on the subject - the key light. I very often use just that, with a reflector for a bit of fill in. You can do a tremendous amount with just one light, preferably with a nice big softbox
You might want to use a second light for fill-in, or a third one for an effect, such as a hair light from the side or behind. A hot shoe gun can often be used for this as you usually don't need much power.
Then you'll want to do something with the background, which is effectively a separate subject. Again, you can do a lot with just one light but if you want the blown white look, you'll need two.
The other thing to consider is that the price of two or three light kits is much better than buying individually. Like Dave, I think three is a good number, especially if you've got a hot shoe gun you can work in when needed (probably not often).