Can a knight fork 8 pieces at once?
Assuming a cooperative opponent, you could have it fork 8 pieces, yeah. But not if the King was involved. Out of the 8 squares that a knight attacks, it must have moved from one of those squares into its new position in that turn. That means there could not have been an opposing piece there for it to fork a full 8 pieces. But, a cooperative opponent could move an 8th piece into that position in their turn. If the King was involved though, 7 would be the max. If the King was one of the initial pieces forked, then on that player's turn, he'll have to move the King out of check, so the Knight will only be forking 6 pieces. If the King was not one of the initial pieces forked, it is not allowed to move into check so would be unable to move into the square the Knight was in originally, to become the 8th target. Lastly, a knight can never normally fork both bishops. All 8 squares that a knight attacks, are the same colour. So since one of a player's Bishops occupies a white square, and one on a black, it's not possible for both to be forked by the same Knight. I'm ignoring a pawn promotion, for which a third Bishop could be added if the player chose one instead of a Queen (a player might want to do this if promoting to Queen would cause an immediate stalemate)
That would require either an unbelieveable struck of luck, or extremely poor skills of your opponent.....and even then, you can only fork 7 pieces MAX, as YOU just vacated the 8th fork. ....by moving into position for the 7 way fork. I've had some situations that led to a 3 way fork, or maybe a 4th......but 7, would really be quite extraordinary. That's a pretty crowded board.........and I would have to wonder, what pieces YOU are leaving exposed, focusing on such a trap.
Unfortunately, a Knight as only fork no more than 7 pieces at one time. This is because that 8th spot was previous location of the knight & therefore will be completely empty. It would be extremely foolish for your opponent to move a piece into that square to make it an 8-way fork. There's probably some other sanity checks involved here as well to make such a scenario extremely rare to happen. The most you may get up to would be a 4-way fork in a game under more ideal conditions.