Legally, there is nothing to stop you specifying in your lease agreement that the apartment may only have a single occupant - but you would probably achieve a better rent by allowing a couple which should well cover the cost of the additional wear & tear of a second person.
I your PRTB registration of the tenancy, there is a requirement to state the number of bed spaces, but how they define a bed space, I don't know but would presume that a double bed would be 2 spaces.