Whatever shares you have at the time of the split you will get an equal amount of Arconic. You will also receive 80 % of the "new Alcoa" with Arconic holding the other 20% this is how the deal is tax free. You end up owning the same proportions as you did before the split.
In listening to the C.C. this morning I believe I heard the following. Say you own 1,000 shares of todays Alcoa when it splits you would receive 1,000 shares of Arconic and 800 shares in the "new Alcoa". I say this because Klaus talked about Arconic keeping a 20% stake in new Alcoa. Did anyone else get that ??