وَمَا لَيْسَ لَهُ عَيْنٌ مَرْئِيَّةٌ فَطَهَارَتُهَا أَنْ يُغْسَلَ حَتَّى يَغْلِبَ عَلَى ظَنِّ الْغَاسِلِ أَنَّهُ قَدْ طَهُرَ.
Translation: If impurity has no visible substance, its purification is washing until the person washing has a predominant conviction that it has become pure.
Predominant conviction is a grounded judgement after adequate washing, not a baseless guess. With invisible filth, the stated criterion is enough washing to become reasonably convinced the impurity is gone.
This passage does not stipulate one fixed count for every such case; it ties the result to well-founded conviction.