Glad to hear that you are interested in Servicecamp.
Here are answers to your questions.
There are several ways for submitting tickets to your IT support. You can use default email address of Servicecamp and send mails to that address which will be converted to tickets.
You can configure your own mail account (like firstname.lastname@example.org) in servicecamp and mails send to your email address will be converted to tickets.
There is User Portal available and people who has link of user portal can submit tickets from there. Recently we released integration with QS module that allows to open tikcets directly from QuickSupport module.
So basically everyone including your employees , customers or partners who has email address or URL of your user portal can submit a ticket. Please note that you can have several email address, for example one can be used by your employees for internal requests and the second one can be used by your customers.
There is now such meaning as quantity of servicecamp. You can have one servicecamp instance and then add you IT support personal as staff agents in that servicecamp instance.
I would suggest to register a servicecamp trial and play with it a bit to get more insight on how it is working.