Is the athar that Abu huraira would make istighfar 12,000 times a day authentic? And is it allowed to specify a specific number to do without thinking it is sunnah such as 1000?

Ustadh AbdulAziz al-Haqqan

Allah knows best. Yes you can make istighfar or a thikr for more than a 100 or a 1000 no problem.