When washing something to remove najaasah (impurity), is it stipulated to wash it a certain number of times in order to purify it?
Question

If children’s clothes are najis (impure), then they are put in the washing machine, is it sufficient to wash them once, or must they be washed two or three times?
Praise be to Allah.Human urine is impure (najis) according to scholarly consensus, as was narrated by an-Nawawi, Ibn al-Mundhir and others,
See: al-Majmoo‘ (2/567).
There is no text to  indicate how many times something is to be washed in the case of najaasah (impurity), except the najaasah of a dog only; the najaasah of a dog is to be washed seven times, one of which should be with dust. As for other kinds of impurities, no specific number is stipulated; rather they must be washed until the impurity is removed, even if that is by washing it once.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: The correct view is that it is sufficient to wash something once if that will remove the impurity and purify the place, except in the case of a dog, which is referred to above.
If the impurity is still there after washing it once, then it should be washed a second time, and a third time, and so on.
End quote from ash-Sharh al-Mumti‘ (1/421).
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Does laundry become pure when washed in an automatic washing machine that washes things by itself without any human involvement?
He replied: What appears to be the case is that the questioner is referring to when a garment contaminated with impurity is washed in an automatic washing machine that runs on electricity; does it become pure or not?
The answer is that it does become pure, because this water fulfils the purpose of removing the impurity. What we mean by removing the impurity is removing its substance by any method, even if we assume that the individual hang out his garment on the roof, then rain falls on it and purifies it; in that case it becomes pure, because there is no stipulation that a specific intention (niyyah) be formed to remove impurity.
End quote from Fataawa Noor ‘ala ad-Darb.
It says in Fataawa al-Lajnah ad-Daa’imah (4/196): If garments are thrown into pure water and washed until the trace of the impurity disappears from them, they all become pure, whether from the najaasah of a dog or otherwise, on condition that in the case of the najaasah of a dog, they be washed seven times. End quote.
Shaykh ‘Abd al-‘Azeez ibn ‘Abdillah ibn Baaz, Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd.
If an impure garment is washed in a washing machine and all traces of the impurity disappear from it, then it has been purified thereby.
And Allah knows best.