Rabbeinu [Tshuvos 1/20] makes a chakirah: Is the איסור for a king to marry more than 18 wives on the מעשה נישואין or to have more than 18 wives.
Nafka Minah: If he married the women before he became king. If the איסור is the מעשה נישואין then he can keep them because he did no מעשה נישואין באיסור. But if the איסור is not to have more than 18 wives then he must divorce his wives over the 18 mark.
Another Nafka Minah: When he marries his 19th wife is only the last wife forbidden to him [because he did a מעשה נישואין באיסור] or are all of the women forbidden to him equally [because now they can all be considered number 19].
The Chinuch says to divorce number 19, implying that SHE ALONE is the חפצא דאיסורא. Otherwise he should have said to divorce any one of the women. Nafka Minah #2 is resolved. Only #19 is forbidden.
Based on the Chinuch - we can also resolve the first Nafka Minah. Since only wife #19 is the חפצא דאיסורא and the איסור is not to HAVE 19 wives but the מעשה נישואין - if he became king with more than 18 wives he may keep them.