UNIX equivalent of windows terminal server options?

I want to replace Windows terminal server mostly due to cost reasons license cost for 2100 users goes out of roof.

The end-user is all windows but I want a jump server that is UNIX based , I have some experience with VNC but I don't want options exists in UNIX to run a terminal services for 2100 users. Some of the basic requirement for such a setup be:-

use of browser to access web based application
Restrict or limit desktop application based upon user authorization.
Supporting concurrent users for about 1200 per server.
Please let me know how should I go about.

So you want to run a whole bunch of web browsers over VNC, to access server-local content with?

"restrict or limit desktop application" is an OS feature, not a VNC feature.
"supporting concurrent users" is a hardware and memory thing, not a UNIX thing.

server content is an CRM application, it is code that decide effective use of hardware resources esp at peak loads. Also, I'm not focused on VNC, I have asked for any "windows like terminal services". Thanks.

What is this "an crm application"? The server being able to actually run and serve whatever it is, is an important consideration...

Oracle CRM

Oracle usually demands their own proprietary server for their software products, no?

No its we based application. Code rest at server side.

There are far simpler and less hardware-intensive options if you consider that possible - a proxy or VPN, for instance, can be an authorization wall without needing to host what's effectively two thousand video streams.

If you really want the brute force way, lots of bandwidth and lots of memory is suggested.

There is a VPN option already available and used the reason for JMP server is running a security agent for monitoring app behaviour.

What open source pkg should i use?

Running a security agent? You need to be able to peek on all these sessions, then?

exactly.