What they do in the Elimination Chamber match is replace the commentators with the chamber cage, and even then the game used to run significantly worse in EC matches than normally. They could probably get 4vs4 matches with a ref running without too many performance issues, basically, but I'm guessing a lot of their engine is hardcoded around the current limit.
They should absolutely be able to get 8 men in the ring at once in Rumble matches, but maybe they don't want to because it'd be too crowded.
They could do it as long as their engine isn't limiting them. Royal Rumble on DC was enjoyable only because of the amount of people in the ring.
Some poor move decisions still by the looks of things in this EC video. Unless I missed something, I don't think Cena has ever done a half nelson throw into a neckbreaker.