I have done 6 in the past, so I see no reason Magnus cannot do 66 with some practice. But I don't think he will attempt something like that without considerable financial interest.
Also just out of curiosity , can you still do blindfold now ?
Yes of course, but certainly at much lower level.
Last time I played was 7 years ago against 4 players level ~1600, and it went 2-2. Now I would almost certainly end with a minus.
Some people can hold a lot of different things in their mind at the same time, others need to concentrate more narrowly. Just the strength of a player's performance in normal mano-a-mano otb games won't necessarily predict their simultaneous blindfold capabilities. Naturally very strong players will win more of the games they play, but it may be that the world champion can't keep as many different games straight in his mind as some IM does.
George Koltanowski, former onetime champion of Belgium and longtime San Francisco Chronicle chess columnist, set a world record (since broken on many occasions) of 34 (24 wins, 10 draws). He also set a record by playing 56 consecutive blindfold speed games (50 wins, 6 draws). the 34 games in Edinburgh Scotland were more remarkable as some of Edinburgh's strongest players participated.
Some masters play large numbers of simultaneous games or blindfold simuls just to set records, get publicity, and collect the entry fees. Often they don't do that well in winning %. GM Timur Gareyev holds the world record for successfully playing 48 games (scored 38.5 vs 1700 elo average rating). Magnus Carlsen beat 10 average-strength Harvard lawyers, but vs 10 stronger members of the Oslo Chess Club (including Aryan Tari--later World Junior Champion) Magnus only scored 7.5-2.5.
So it would seem that blindfold chess is not one of Carlsen's strengths, making it unlikely that he would be able to complete 66 simultaneous games.
I was able to do 4 against weak competition about 30 years ago. Won all 4 twice. I didn't try for more, there were only 4 sets. I was 1800ish then.