What is the Rebbe’s Minhag in how many Haddasim to use? Is there a specific reason why Chassidim shouldn’t use that amount?
It is the Chabad custom to add multiple Haddasim to the standard three, adding at least another three. The greater the number of additional Haddasim, the better.
In Sefer HaMinhagim page 65 it says: 4, 12, 13 or 26 but not 9, 68 or 69.
The Rebbe’s personal custom is to take 36 since 5741. (In 5711 – 13; from 5712 till 5740 – 26).
I don’t see any reason to use or not to use the same amount the Rebbe does.
It should be noted that unlike other customs, in this specific matter, the Rebbeim give specific guidance to different individuals how much they should take. Also, the Rebbe himself changed the amount after the nesius. Same with the Rebbe Rayatz.
Whatever amount one uses is okay so long as one can afford it and it does not come on expense of Simchas Yom Tov of the family.
ראה אוצר מנהגי חב״ד ע׳ רעג. וש״נ.