CSoket & Multiple users

Hi, all.

While I took TCP/IP class, I had the socket programming.
So, I developed Bank Client and Server by Dialog app - MFC.
My professor told me that you can make your own software company.
But I can't believe him?

Now after I finish this class, I try to update my app during this break.That is, support multiple users and database like Access.
Server app does the access to database to read account #, password,
transaction and balance.
Because Access is a desktop database, there is possibility that
Server app will be stopped.

Does anyone has an idea of how many people will be able to use this
app at the same time?
That is, I guess that I must control the situation that Server
reads the socket and send this info to the database.
If someone acutally develop this kind of application, how do I do the
test of this? I mean that I can't do this test because I can't act
1000 customers?

Thank you in advance.
-Masaaki Onishi