Rev 7:8 Of the tribe of Zabulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand.
Rev 7:9 After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
John saw the 144000 sealed, then said "after this I beheld lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues"
Plainly seperate groups of people, the second group is those raptured.
Rev 7:13 And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they?
Rev 7:14 And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.
John didn't know who this group was, the angel told him, John knew who the 144000 were, he witnessed their being sealed.