[rentj]

For example, in toluene, a monosubstituted benzene ring, all of the aromatic hydrogen are chemically equivalent since only two signals are shown in proton NMR. Does this occur to all monosubstituted benzene ring and what is the logic behind this? Thank you.

[Borek]

all of the aromatic hydrogen are chemically equivalent since only two signals are shown in proton NMR

Not exactly. Whether they show as equivalent or not depends on the instrument resolution (so technically they are never equivalent).

[Babcock_Hall]

Spectra of aromatic compounds look very different on old 60 MHz NMR instruments than they look at, say, a 400 MHz NMR instrument.  Have you tried applying any rules that you learned to monosubstituted rings?  If so, what did you learn?