What transistor should I use to power my 12v Display and control with a 5v raspberry Pi
Any common small signal transistor would work in your circuit as you shown. Assuming each segment only needs 20 mA to 200 mA, a 2n3904 would work.

Though Your diagram has the transistors oriented wrong. A npn transistor has the emitter on the low side with the arrow pointed towards ground. Flip them around.