I write this on informed authority (friends and clients who are cops). First of all, the issue is not how many women work in an in-call/brothel situation. The legal issue is, "is this a place where prostitution services are REGULARLY provided. In a bust situation, the cops will send in three under cover "johns". If they are promised a sexual service and there's an exchange of money, then it's a legal bust. The cop(s) do NOT have to go through with the call although some do and then perjure themselves for sake of saving grace/looking credible. It's the offer that makes it an offence. Once the offer's made and the cops can prove that it's not a singular event then the lady can be busted for keeping a common bawdy house. This is why some ladies will use hotel rooms and change regularly. Sort of like a floating crap game. If she can't be proven to be regularly providing sexual services in one place, then she can't be found guilty of keeping a common bawdy house.
In terms of the swingers clubs or excessively sexually active adults, that is not a common bawdy house because there is a lack of financial exchange. There has to be an exchange of money with a promise of sexual services for that money. That's why swingers' clubs that advertise, like Club Privie for example, specifically state that the membership fee you pay to be invited to their parties is for membership into the club, not for a promise of sexual services.
As for the enforcement issue, I have been told by at least four Toronto cops that they are not interested in shutting down independent service providers. The only time they investigate someone is when it comes to their attention for other reasons: immigration issues, underage girls, drugs, excessive noise complaints from other tenants, etc. One place I worked at in downtown Toronto drew attention because the four or five girls working there were ordering food from delivery services at unusual hours and the noise and traffic involved made a noisy neighbour suspicious.
Having said that, I have been told by cops that they don't have the same attitude towards agencies that are running in-call services because they see that as predatory (agencies making money from the labour of others). As well, a lot of agencies are into large profits that are usually unreported to Revenue Canada which is seen as another incentive to close them down.
Another factor, too, is where you're operating. Toronto cops are less concerned than say Durham or Peel. Where you operate can be an issue, too. If you're using a house in a residential area and a neighbour suspects something's up (as in the case of the Madam in North York who was into domination), that can sometimes be your downfall. A cop I know told me that it is easier to put a house under surveillance than it is an apartment which makes their job easier.
Another issue is public visibility. The police like to be seen to be doing their job. I used to have a client who was a photographer with a newspaper. Once or twice a year, he would get a call that the police were going to bust a massage parlour or make random calls to in-call services. He was asked to be on stand-by to take photographs of those being arrested. He would tip me off by telling me not to answer the telephone that night.
The current political will, in Toronto at least, is that out of sight is out of mind. Streetwalking is seen as a nuisance and so therefore a justifiable target. If an independent lady is running a clean business and isn't drawing negative attention to herself, there should be no problem for her. That's not to say that it won't be the same somewhere like Mississauga or Oshawa. Every locale has it's own perceptions of this business.
Hope this has been helpful!