Just to be clear, this isn't a question about whether there were ten, eleven, or twelve in the group, but rather how certain we are, perhaps based on the syntax of the original Greek/Semitic, that "and then to the Twelve" in 1 Corinthians 15:5 is a reference to an appearance of Christ to the apostles all at once, as opposed to a series of individual appearances to the rest of the apostles besides Peter. Perhaps this is an odd question, as I've never seen it asked before. Is there any dispute about this among New Testament scholars?
What I think you are asking
"...is a reference to an appearance of Christ to the apostles all at once, as opposed to a series of individual appearances to the rest of the apostles besides Peter." Specifically related to this verse.
(1Co 15:5) And that he was seen of Cephas, then of the twelve:
I am going to go with phraseology of "...and then to the Twelve", mainly because you also suggest "...perhaps based on the syntax of the original Greek/Semitic."
Here is what Gill says on that matter, and he directly addresses "the twelve", as an idiom of that day.
"then of the twelve; though there were then but eleven of them, Judas being gone from them, and having destroyed himself; and at the first appearance of Christ to them, there were but ten present, Thomas being absent; and yet because their original number, when first chosen and called, were twelve, they still went by the same name; see Joh_20:24. The appearance or appearances here referred to are those in Joh_20:19. The Vulgate Latin reads the "eleven"; and so the Claromontane exemplar (Gill, John, D.D., (1697-1771) Pub (1746-1766), 1816; public domain)."
see (1Co 15:5)
In referring to the following (Joh 20:24) below; Gill is attempting to validate the idea, that "the twelve" was taken to be a 'part for the whole' type, expression (synecdoche), in common use, at that time (idiomatic). Gill makes reference to the Codex Claromontanus, this is the codex that Beza found, not Beza's translation. Gill usually quotes Beza. He appears to be providing the more reliable, agreement with the Vulgate.
The point made by referring to these; the more exact intention, given in the same place, and in the same way; suggests that the original could have allowed for that type of expression; only, makes a correction for their number. '...Latin reads the "eleven"' The point would have been clearer had he included the article in the quotes, as he obviously supplies it here.
(Joh 20:24) But Thomas, one of the twelve, called Didymus, was not with them when Jesus came.
I supply the other verse (John 20:19). The account that Gill accepts, as the instance referred to by Paul in (1 Co 15:5); your question, I think.
John 20:19 Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.
Other things to consider
The fact that Paul is giving an iterative account here, suggests error if we refuse the order. In keeping with the order, as he gives it (1Co 15:5) first "seen", 6 "After that...seen", 7 "After that...seen", 8 "last of all ... seen"; Gill is correct, it is the first account. just as--
(1Co 15:7) After that, he was seen of James; then of all the apostles.
Must also correspond to--
(Luk 24:50) And he led them out as far as to Bethany, and he lifted up his hands, and blessed them. (Luk 24:51) And it came to pass, while he blessed them, he was parted from them, and carried up into heaven.
This being spoken of by Paul, as the last; because the following is final--
(1Co 15:8) And last of all he was seen of me also, as of one born out of due time.
Whether he refers here to the Damascus road encounter( most likely), or special revelation; being caught up; the only relevance here, is that it was "...out of due time." He is referring to his Apostleship, so that, we can't take it to mean, that he had seen him risen, before his conversion.
Order is typically a courtesy of Paul. I believe that Gill has it correct, and for the reason he gives. Though, the existing emphasis of order, also suggests, that he is referring to specific instances; first, and then, and then another, and then finally.
I hope that this was your intention.
We know from John's account that Thomas, at least, was not present when Jesus first appeared to the other Apostles:
But Thomas, one of the twelve, called Didymus, was not with them when Jesus came. The other disciples therefore said unto him, We have seen the Lord. But he said unto them, Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe (20:24-25).
If you are asking if there is anything in 1 Corinthians 15:5 whether Jesus appeared to all or some Apostles separately or in a group, I don't think so, but why would one take only that particular Scripture in isolation without considering other accounts?