Does Dixit need any modifications for more players?
You need some way for each of those players to secretly bid on what card they think is correct (numbered playing cards could work). As for the number of players affecting how likely it is for a random guess to give them points, let's do some math and find out.
Let's assume in all these examples that the storyteller's card is card #1. Player2 plays card 2, Player3 plays card 3, etc.. Let's ignore 3-player alternate rules variant for playing 2 cards. Player's also cannot vote for their own cards (from the rules).
1) 25% Player2 guesses #1 Player3 guesses #1, Storyteller gets 0 points, other players get 2.
2) 25% Player2 guesses #1 Player3 guesses #2, Storyteller gets 3 points, Player2 gets 4 (+1 from Player3's wrong guess).
3) 25% Player2 guesses #3 Player3 guesses #2, Storyteller gets 0 points, other players get 3.
4) 25% Player2 guesses #3 Player3 guesses #1, Storyteller gets 3 points, Player3 gets 4 (+1 from Player2's wrong guess).
This works out to an average score of the following:
3-Player Storyteller 1.50 points, Non-Storyteller 2.25 points.
Not to bother you with the math for the calculations, did this in a spreadsheet. (x points/y outcomes).
4-Player Storyteller 2.000 points (54/27), Non-Storyteller 1.630 points (44/27).
5-Player Storyteller 2.039 points (522/256), Non-Storyteller 1.496 points (383/256).
6-Player Storyteller 2.016 points (6300/3125), Non-Storyteller 1.400 points (4374/3125).
My guess as to the average results for 7, 8, 9, and 10 players is that the Storyteller's average points will approach 2 as the number of players reaches infinity, and the Non-Storyteller's average points will approach 1 as the number of players reaches infinity.
A way to curb the Storyteller from just giving bogus clues, would be to give out bonus points depending upon the number of players. In a 6-player game, approximately 40% (1280/3125) of the time only a single player will get the answer right. If you give out 5 (number of guessers) or 6 (number of players) bonus points to the single guesser that got the answer right, this will add an additional 2.05 or 2.46 point on average to the Non-Storyteller players. This should discourage giving bad clues, since employing a bad clue strategy will on average put you 2 to 2 1/2 points behind. This should scale well, because as the number of players increase, the number of bonus points increases. If you do need to extend this further (not sure you need to, but I haven't worked out the odds of only exactly one person getting it right for 7, 8, ... players), you could give out bonus points rounded down for the total number of players that got it right. (example, 10-Player game: if one person gets it right +10 points, if two people get it right +5 points each, if 3 people get it right +3 points each, ...)