MTS ?

For over 500 users, do I have to use MTS? Can I just increase the number of process, like 1000, 2000 ...?

Do you have to? No. Depending on the resources of your server, you probably should with 500 concurrent users.

Dedicated server mode reserves X bytes of memory outside the SGA for each connection whether it is active or not. In MTS, this memory gets allocated from the SGA, but can be shared between users.

If you have plenty o' ram, then feel free to use dedicated servers.

Depends upon the resources on the box. If you have lot, you don't care. If you don't, you do care and alternative is MTS.

You have to verify two things:

1) How many concurrent users are there, and max UGA memory for these concurrent users.
Whether u can save any memory.
2)Scalibility, whether your systemcan handle increasing number of processes and context switches.
3) Ver of your Oracle does not have any bugs associated with MTS enabling.
4) You want to use Connection pooling, EJB, or Advanced Networking option.

Take Care
GP