Integer type question
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Here P is the benzene and Q is acetaldehyde(oxidized product of ethanol).If it is introduced on benzene ring ,it will give acetophenone which can give only one monochlorinated isomer as it has only one methyl group which can undergo chlorination.

The answer is it will give one monochlorinated isomer.
Explanation:
                       CH3CH2OH         
C6H5ClN2                   C6H6+CH₃CHO .P = C6H6, Q = CH₃CHO
2C6H6+CH₃CHO  + C6H5COCH3
R = Product of P and Q = C6H5COCH3 known as Acetophenone.
As the name suggests monochlorination refers to chlorination with a single atom even in cases where multiple chlorinations are possible. We can find number of monochlorinated products by findingthe number of structures drawn. Acetophenone has only one methyl group that can undergo chlorination so it can only give one monochlorinated isomer.