1. Does total number of offline users have an effect on the performance? Do we really need to plan for performance for offline people?
No, offline users shouldn't affect performance.
2. Can having too many communities affect the system? ~200 communities..
You'll definitely want to up the cache size (you can do this via the admin console) for the community cache and there are a couple places in the application where having a large number of communities causes an awkward UI, we're working on that for 2.0.
3. Is clearspace read heavy or write heavy assuming people discuss, blog and write docs?
Totally depends on your community. Internal or external? In general, I'd guess that an internal community would have more writes, you get more lurkers on an external community.