Then there are the hackers, who'll build their own units or modify them to do new things. My hookerbot will have lasers from Blu-Ray drives in her eyes so she can pop balloons from a distance when the stimulation counter hits the maximum value.
Ah, I can see the first murder trial, murder through hacking the sex bot into killing the owner now!
Anyway, as long as they remain non-sentient I don't have a problem with them. The moment they become sentient though, they are alive and should have rights like any other one of us.
And here's rather the rub. For example, to make a sex bot be convincing BDSM slave, she'd have to simulate feelings of pain while at the same time enjoying the pain. She'd want to be away from the pain - especially as it's being inflicted - yet at the same time crave it, desire it, enjoy it. She'd want to back away some times, eagerly receive it the next - also depending on how hard you let the whip go, and such. The result is an incredible amount of sensors in the skin, and a computer brain to react to it; proper simulation would also require more sensors in more sensitive areas. In essence, the computer would have to become aware of her body and the world around it - the very definition of sentience. To get ever better a simulation you would eventually very possibly create an actual sentient being.
Indeed, a BDSM sex-slave android, may very well be the first android to achieve sentience. We should really think about how to recognize (emerging) sentience in computers and androids, and also a way to make sure it doen't happen anymore if we wish to continue to have sex androids that are not unethical and amoral.
As for there never being more than 100, yeah, sorry, but, no way in hell. If computers keep progressing in power and storage capacity the way they are now, the same thing will be happen to the androids. You won't buy one and always keep it; there'll be a newer computer system, a more advanced body with better and more sensors to be more sensitive and to react more like a real person. When the reach buy a new one, what'll they do with the obsolete model? Trade it in for a discount on the new one and then the seller will re-sell, or will they sell it to someone else themselves? And so, you have another person with and android, and so forth, and so on. And a couple of decades later the thing will sell for near to nothing.