Which symbols are used to represent different Bohr's orbit?

there is no different orbit, only different energy level

The orbit with lowest energy level is n=1, and as energy increase, we have n=2, n=3 and so on. 

Bohr's model describe atom as negatively charged electrons move around an positively charged nucleus. which is similar to a solar system but it is hold together by the attraction force instead of gravity. 

So the area which the electrons moves, is called an orbit, different electrons have different energy so it moves in different orbit. 

For Bohr's model, orbits are only specified by difference on energy level, and there is no symbols to represent different orbits. 

Just n=1, n=2, n=3 and so on.