If a fly sits on napaak thing and thereafter it sits on clothes which are wet, then will the clothes become napaak? Also, if there are many flies like that, and we don't know which one sat on the napaak thing then what should we do?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
Because it is virtually impossible to identify which fly has sat on what, this will not change the ruling and the default ruling of purity will apply in all cases.
Only Allah knows best
Written by Maulana Imran Mughal
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 اليقين لا يزول بالشك.