Ibn Katheer says in his tafsir for 56:11 that As-Suddi said that the people of the right hand comprise the majority of the people of Paradise. I am wondering what is the evidence for that? If it is true, does it mean that most Muslims will not go to Hell?
Also, is there any evidence for what portion of Muslims will be punished at all, or in the grave?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

The statement of As-Suddi that the people "of the right" comprise the majority of the people of Paradise means that they are more in number than “the forerunners” (i.e., the foremost in faith).

The people of Paradise are two categories:

The first category are the forerunners, and those the ones are brought near to Allah; Allah Says (what means): {And the forerunners, the forerunners -. Those are the ones brought near [to Allah]. In the Gardens of Pleasure. A company of the former peoples. And a few of the later peoples,} [Quran 56: 10-14]

The second category are "the people of the right", who are the righteous, and they are the majority of the people of Paradise, as evidenced by the verse in which Allah Says (what means): {A company of the former peoples. And a company of the later peoples.} [Quran 56: 39-40]

A company means a large group. The forerunners in the later people are few, and the people of the right among the later people are so many. This proves the large number of the righteous people who are the people of the right when compared to the forerunners. This is what As-Suddi meant with his statement.

This is not about the fact that most Muslims will enter paradise and will not enter Hell at all. Whoever dies as a Muslim, his fate will be Paradise undoubtedly. There are many Ahadeeth that are confirmed with Tawaatur that some disobedient people among the Muslims will be tortured in Hellfire because of their sins and then will come out of it, either by virtue of the intercession of the intercessors or by virtue of the mercy of Allah The Most Merciful.

None of the people of Tawheed (i.e. the Muslims) will remain in the fire forever [rather they will come out of it after being purified from their sins].

For more benefit, please refer to Fatwa 264145.

Allah knows best.