The blessed Qur'an makes mention in Surah al-Mu'minoon;of two categories of women with whom men can have intimate relations.
The first category is wives, and the second is Millk al-Yameen (i.e., those whom your right hands possess).
The Qur'an does not define the second category of women as slaves.
In Akhir al-Zaman very large numbers of women will not be able to find husbands to maintain and protect them as well as to provide for intimate relations.
In consequence of Dajjal's brain-washing, most men will marry only ore wife, and they will be unwilling to accept women as Milk al-Yameen.
In fact so few men will be prepared in Akhir al-Zaman to assume such responsibility to women in need that the ratio will be one man having to maintain and provide for 50 such women. Most men will therefore have to answer on Judgment Day for having abandoned their responsibility to accept, either as wives or as Milk al-Yameen, the multitude of women in need of men.
The Law of Necessity can be invoked in Akhir al-Zaman to permit a (free) woman to offer herself as Milk al-Yameen to a (pious) Muslim man of her choice, and if he accepts her offer, he would then assume the responsibility of maintaining and protecting her. The Qur'an also gives them both the right to intimate relations with each other.
Milk al-Yameen should be referred to as a wife - but not of the same status as a wife of a contract of Nikah. For example, she does have to be maintained on parity with a wife, nor does she enjoy inheritance rights.
The limit for the number of MY that a man can have is determined by his economic means to maintain them.
Neither men nor women should ever enter into a contract of MY with a Niyyah other than that of a permanent relationship (as in a contract of Nikah)..We must recognize, however, that in the same way that circumstances sometimes cause a divorce to take place, so too can a contract of MY sometimes be terminated or upgraded to a contract of Nikah.
Questions concerning the law of Milk al-Yameen should be posed to any Mufti (who all have to study this subject in their legal studies).