> how many user can use my application with a database .gdb?

As far as I know, IB doesn't have a restriction on the number of user's that
can connect to the database.
This number also depends on the network infrastructure, database design,
application design and server hardware.

> should I use different transaction for every user or single transaction is

enouph?

If not specified explicitely, every users get's a unique transaction while
connecting to the database. It's of course a better way controlling the
operations on explicit transactions, but does your application need it?