Yes it's a brothel, as is anywhere where more than one prostitute works whether they are doing so on different days or not. Whether or not anybody will care depends a lot on where it is.
If the girls work on different days, would you be in trouble with the police? As on the surface it's one girl working?
Well these are really two different questions

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It doesn't matter whether the prostitutes (and they don't have to be women) are all there at the same time or all on different days - a brothel is a brothel. That doesn't mean anybody will be in trouble with the police unless they take an interest in it - there are parlours and working flats all over the land which have been there for donkey's years, just as there are some which last a day and a half before being shut down. It mostly depends on who, if anybody, they annoy.
Either way, working in a brothel (or visiting one) is not illegal. Managing (including in being the name on the lease) or assisting in the management of a brothel is, which is why doing either of these things (including answering the phone to take bookings for others when they're busy, for example) is the least desirable position to be in should anybody decide they are annoyed.