Very few. The overwhelming majority never saw an IDF soldier & left because their own leaders told them to "so it would be easier to the push the Jews into the sea" (their own words). The Arab leaders spread rumors of rapes & attacks to scare them into leaving (so fighting would be easier.)
The number who displaced was about 700,000, which is less than the around 800,000 Jews kicked out of Arab lands often without their belongings or money & were absorbed by Israel.
Post '48, Israel reaccepted about 30,000 (I've seen widely varied numbers) of Arab Muslims for the sake of reuniting families. However, since most were hostile to the country it was impractical to absorb them. In Jerusalem after '67, Arabs were given a choice of Israeli citizenship siging an allegence oath or Jordanian citizenship with Jerusalem residence papers. Most took the later. In the years after '48, it was Menchem Begin who did the most to integrate the around 20% of Israeli citizens who are Muslim Arabs (not in the terrories which Israel didn't have at the time) into Israeli society & legal status. (So when people complain about him - this is something to remember.)
Also, post '67 when Israel had the terrorities (agained in defensive war when 5 Arab nations attacked), Israeli jobs were better than Egyptian & Jordanian, so a number of the refugees came INTO the terroritories from those countries at those times to work in Israel. (Thus giving a sense of how complicated politics is in this region.)
From '48 to '67 those areas were under Egyptian & Jordanian rule. Egypt keep Gazan's at gun point in refugee camps. Arabs who went to Lebanon were also kept under gun point in camps there. The local Arab Muslims did not have respect of these other countries & were kept as refugees by their brethren to rile them up & bolster the PR for the war to wipe out Israel.
There is no such thing as Palestinians. It was invented by Arafat in the 1960's to create a sense of "separate people who were displaced." It basically says so in the PLO charter if read all the way through, that they are brethren with & part of the same group as every other Arab. Until then it was the JEWS who were called Palestinians in regional & international papers. The overwhelming majority of Arabs in Israel & in the terroritories came from neighboring countries only a generation before AFTER the Jews started developing up the region, from East Bank (now Jordan), Egypt, Syria, Lebanon.
The goal of the PLO/Fatah & Hamas & of all the other littlier gang groups trying to run things there, is to create a greater regional empire from Syria to Jordan to Egypt. That is what was "stolen" from them by sticking a non-Arab-Muslim country in the middle of their goal. It's all over their websites. For a while they used the world "occupied" but then people caught on that they mean Israeli's existence is occupying what they want. Jews have lived there for over 3000+ years from before Arabs from Arabia invaded. So they are a minority in the Middle East & have a right to a tiny sliver of land. Taking a step back to the bigger map - it's obvious how little Israel is, & how much it can not be about that sliver but about a bigger ideology.
Most of the threat & complication right now is from Iran. Iran is funding on all sides of Israel, Hizbollah, Hamas, & Syria. As a result there is a split even in Palestinian Arab area were Hamas & Fatah are killing each other daily. Their ideology is very similar, with the main difference being how religous to be. Fatah is more secular. So, what they do agree on, is to wipe out Israel.
Obviously neither Israeli or PA policies will be perfect all the time. However, I firmly feel until we outside stop supporting the ideology of wiping out Israel & crying victim instead of building a future & help them make this ideology shift, there can no be peace. Unfortunately any PAs who are of like mind, put themselves at risk currently for saying so. By risk, I mean their lives.
So there's a start of a few things.