My FM transmitter was designed for a 9V battery, not a 12V battery. Its output transistor will have a current that is too high for a little 2N3904 and will overheat.If so, may i supply from 12 v, 45 amp. large battery to your 3904's transmitter? I have no idea to do such equation.
The high available current from a battery does not force all its current into a circuit.
My car has a 12V 840A battery. The starter motor draws 840A when it is cold. But the clock in the car does not use 840A, it uses only 1mA (0.001A).
With a new 9V battery, the output transistor in my FM transmitter draws about 50mA. Its collector load is an LC tuned circuit so its collector voltage is an average of 9V. Then it heats with 9V x 50mA= 450mW which is very warm.
If it has a 12V supply then the output transistor will probably try to conduct 200mA which is too high for a little 2N3904 transistor. It will heat with about 12V x 200mA= 2.4W so a power transistor with a little heatsink will be needed.
You should learn how to calculate electronic equations. It is simple.
You should learn how to design electronic circuits. It is simple.