How many tasks are too many?
The general answer is "Measure, Measure, Measure" :) if you're not experiencing any problems with performance, you shouldn't start optimizing.
I'd say 200 tasks are fine though. The beauty of tasks compared to threads is their low overhead compared to "real" threads and even the thread pool. The TaskScheduler is making sure all the hardware threads are utilized as much as possible with the least amount of thread switching. it does this by various tricks such as running child tasks serially, stealing work from queues on other threads and so on.
You can also give the TaskScheduler some hints about what a specific task is going to do via the TaskCreationOptions
If you want some numbers, check out this post, as you can see, Tpl is pretty cheap in terms of overhead:
.NET 4.0 - Performance of Task Parallel Library (TPL), by Daniel Palme
This is another interesting article on the subject:
CLR Inside Out: Using concurrency for scalability, by Joe Duffy