If someone wants to repent for committing multiple acts of shirk, blasphemy, apostasy, and kufr, can he just ask Allah to forgive him for all his acts of shirk, apostasy and kufr, and take ghusl and say the Shahadatayn? Or does he have to mention each act he did in order to repent?

Ustadh AbdulAziz al-Haqqan

I'm afraid that this is a question coming from Waswas, and I think it is likely so. If it is the case, then answering it is just going to fuel the Waswas you should just stop, as the Messenger (sallallahu alayhi wa sallam) instructed.