Hi, I want know the hardware requirements for an application server, dependent of the number of users (around 1500).
Contact your application server vendor.
They should be able to provide this information.
This is kind of a blanket question. Performance is basically a function of 2 variables: available resources and resources as needed by the application (and the server it's running on). The kind of resources needed by your application will depend on the functionality of your application and can be expressed in memory footprint, database accesses, and I/O and CPU capacity consumption.
The real answer to your question can only be found by running your application on your application server under reasonable load, preferebly a siumlation of 1500 concurrent users. Extrapolation of the results smaller scale tests might lead to nasty surprises.