for 400Users what will be the IOPS required ?
if i go with 450GB15K RPM SAS then how many HDD is required?
All users are not created equally so this can vary from user to user, but some guesstimate around 10 per user. The best way to size your IOPS is by using a tool like Liquidware Labs' Stratusphere Fit to profile your users.
A 15k SAS drive is capable of ~175-210 IOPS.
A real sizing from Liquidware or similar is best, but anywhere from 10-20 is a good bet.
Assuming 400 users, you should be expecting around 12,000 IOPs. But doing it all with SAS drives is a bad idea. Use an array that can do some form of automatic sub-lun tiering (or at least caching) to reduce the number of drives.