It all comes down to a number
One man but receiving money = Mistress or Concubine
Multiple men and receiving money = prostitute
Any number of men but NOT receiving money = whore
Mmmm... Mistress and Concubine are not the same thing. A Mistress is a woman other than a man’s wife with whom he has sexual relations. A Concubine is a woman other than a man’s wife with whom he has at least one child by, sometimes living in the same household along with the wife.
Historically, Concubines were usually taken on when a married couple could not conceive a child by themselves and so a Concubine was taken on in order to continue the bloodline, but they were also a means to increase the family size even if the wife was not barren. Even the Bible speaks of some famous names having concubines, although I will leave it to better-read individuals to note which ones did during their lives.
Bingo.