Datasheets typically use a term call "beta" or "HFe" to describe gain".
And this is current gain. The number represents the collector current divided by the base current. Typically this number will be around 10 to 300.
The gain of a transistor with a beta of 100 with a base current of 1mA will have a collector current of 100mA.
If you want any really useful answers, you need to explain a lot more about what you're doing.
Can you post a schematic? Can you tell us why you need to know the number? Do you have a particular transistor in mind? Are you designing a circuit to work to a particular specification? We're playing tennis in the dark with you unless you give a lot of background information.