According to officials reports of United Nations High Commissioner for Refugees, there are 4,835,909 registered Syrian refugees (as of may 2016). I assume the estimate comes from there. That's slightly less than 5 million.
But the claim that "five million Muslims march on Europe" is blatantly false. Many of them are not Muslim and the overwhelming majority of them are registered in Muslim majority countries. The report says:
This figure includes 2.1 million Syrians registered by UNHCR in Egypt,
Iraq, Jordan and Lebanon, 2.7 million Syrians registered by the
Government of Turkey, as well as more than 29,000 Syrian refugees
registered in North Africa.
The report also states that 1,004,345 Syrian refugees are seeking asylum in Europe (between Apr 2011 and Mar 2016). it's worth noting that many of the are not Muslim.
Also, according to Reuters and Al jazeera, Saudi Arabia has taken 2.5 million Syrians but Saudi government didn't register them as refugees because that would degrade them. The report states:
But Gulf states say they have taken in hundreds of thousands of
Syrians since the civil war there began in 2011, just not as refugees.
The source said Saudi Arabia had received nearly 2.5 million Syrians
since the conflict erupted.
"[Saudi Arabia] was keen to not deal with them as refugees, or to put
them in refugee camps, to preserve their dignity and safety, and gave
them complete freedom of movement."
"[Saudi Arabia] gave whoever chose to stay in the kingdom, which are
in the hundreds of thousands, proper residency ... with all the rights
that are included like free health care and engaging in the workforce
The kingdom has also provided about $700m in humanitarian aid to
Syrians and had set up clinics in various refugee camps, the statement
by the SPA said.
The official source said more than 100,000 Syrian students were
receiving free education in the kingdom.