Yes, indeed ... if Israel took over all the land it conquered, it would get SIX MILLION ARABS.
As to what impact they would have, all you have to see, in that regard, is that RIGHT NOW, without Israel taking over all the land, the existing 20% of the Arabs already have the THIRD LARGEST PARTY in Israel. Add in those extra Arabs, and you don’t have a Jewish State any more, but an “Arab-controlled state”!
Here is a blast of reality, exposing the bogus demographic threat that does not exist: http://www.algemeiner.com/wp-content/uploads/2014/04/Yoram-Ettinger-Demography-English.pdf
Here is another piece, this time in article rather than power-point form, making the case quite compellingly: http://chersonandmolschky.com/2015/03/11/arab-womb-fertile-all-israel/
You can propagandize against annexation all night long if you wish, but the facts are the facts and there simply is no demographic threat to Israel that would result from its annexation of Judea and Samaria.
Arabs make up about 20% of Israel excluding Judea and Samaria, and the combined list of Arab parties (the single party that you refer to) makes up just 20% of the Israeli Knesset. The combined list is not going to be able to hang together because the interests of the various factions are at odds with one-another, and as soon as they break up at least one, and probably at least two, of the parties that comprise the Arab list will fail to win enough votes to achieve representation in the Knesset.
Nobody who looks at the facts is at all concerned about the long-debunked demographic time-bomb argument against annexation.