A fuzzy scenario for illustrative purposes, numbers in millions:
104 shares total
Assume another 10 million shares repurchased (approx $450 million?) = 94 shares outstanding
Pull shares from loan to short sellers:
14 million current Icahn position = 80 shares for loan
Assume Icahn position increases to 25%, another 10 = 70 shares for loan
Now assume other longs including Hedge funds pull 20 shares (this number could get much larger) from loan to shorts = 50 shares for loan.
Ackman's short position will be nearly 50% of outstanding shares, an extremely vulnerable and completely untenable position. He must know that even if his short thesis is correct the stock will not pay off and he has to cover. Game set match.
you also need to deduct shares controlled by the BOD and other insiders, and any shares held in retirement accounts incl 401k and IRA -- those are not eligible for lending. Further, you can deduct all of the near term ITM calls written as those guys need to be provisioning shares unless they are covered calls.
Don't forget the roughly other ~13 million short shares by various other shortsellers. Not sure if your post was intended for specifically an Ackman squeeze or a short squeeze in general.
I think Icahn knows this math and I think he is setting up for a VW style short squeeze of epic proportions. I think he has dreams of a mediated sale/cover of his shares to Ackman, after he can't find anywhere else to cover :)