With the present version of WebDirect I would say that 200 simultaneous users will only be possible with a very efficient build and simple database.
A good alternative is to use Custom Web Publishing, CWP, which can easily handle 200 users at the same time.
It all depends on the solution.
Thnx for ur reply Carsten .
Just one more question ... if this number is more than 200 users , the database will be crash ?