The only study I’ve found that directly addresses this question is rather old (1974), I’m afraid. It was sponsored by the DOJ, and was based on police arrest records of 1087 females.

It found that 93 were both prostitutes and addicts, 134 were addicts but not prostitutes, 119 were prostitutes but not addicts, and 741 were neither addicts nor prostitutes. So in this sample, 40.9% of the addicts also engaged in prostitution.

The study further broke down the statistic along racial lines. Almost half of the black addicts were prostitutes, while 20% of the whites were prostitutes.

I don’t know how applicable these statistics are anymore; a lot has changed. Drug use overall has declined, and the drugs used have also changed.

it might be easier to find out the number of prostitutes that are drug addicts.