How many instances is this?

Well, the most recent event that got this conversation going in Europe was the New Years mass sexual assaults in Germany. It involved 2000 migrants, acting in groups to commit 1200 sexual assaults and at least 24 rapes in seven different cities across Germany in one night.

https://en.wikipedia.org/wiki/New_Year's_Eve_sexual_assaults_in_Germany

During the 2015/2016 New Year's Eve celebrations, there were reports of mass sexual assaults, at least 24 rapes, and numerous thefts in Germany, mainly in Cologne city centre. There were similar incidents at the public celebrations in Hamburg, Frankfurt, Dortmund, Düsseldorf, Stuttgart[25] and Bielefeld.[10][26][27] For all of Germany, police report that ~1,200 women were sexually assaulted and estimate that at least 2,000 men were involved, acting in groups.[28]

All of the incidents involved women being surrounded and assaulted by groups of men on the street.[29][30] Police reported that the perpetrators were men of "Arab or North African appearance" and said that Germany had never experienced such mass sexual assaults before.[20][30][31][32][33] The German Federal Criminal Police Office said the incidents were a phenomenon known in some Arab countries as taharrush jamai (translated as "group sexual harassment").[34][35][36] The attacks sparked an international outcry, a debate about women's rights, the sustainability of Germany's asylum policy, and social differences between European societies and those of North Africa and the Middle East. Taking place during the European migrant crisis (see timeline), the attacks also led to a hardening of attitudes against immigration.[37]

Chief Prosecutor Ulrich Bremer stated that "the overwhelming majority" of suspects were asylum seekers and illegal immigrants who had recently arrived in Germany.[38][39] Only a small number of the alleged perpetrators have been identified. By 9 April, police in Cologne had identified 153 suspects, 24 of whom were in investigative custody.[4][5][7][15][40] Almost all of the suspects of the Cologne crimes were non-Germans; two-thirds of them from Morocco or Algeria. 68 suspects were asylum seekers; 18 were residing in Germany illegally, and the legal status of 47 others was unclear. Four suspects were underage, unaccompanied refugees.[7][41][42][43][44][45] By July, four perpetrators had been convicted,[28] and it was reported that half of the 120 outstanding suspects had been in Germany for less than a year,[24] most of them from North Africa.[46]

http://www.bbc.com/news/world-europe-35261988

What happened during the first hours of 2016 is likely to have a profound impact on the rest of Europe.

Certainly the boldness of the assaults and the sense of a powerless state will haunt the victims, but what has also been lost is trust - the essential glue in any society.

There is now a widely held suspicion that the political elite is not being candid with the German public.

There was the inexplicably bland initial police report describing the evening in Cologne as a "relaxed atmosphere. Celebrations largely peaceful". It was on social media that news of the assaults first seeped out.

It took the better part of a week to acknowledge that asylum seekers were among the suspects.

The police certainly knew the reality of who had been on the streets. On the night some young men had shown police their asylum documents.

An internal police report describes a man telling the police: "I am Syrian. You have to treat me kindly. Mrs Merkel invited me".

What has fuelled the sense of crisis is the suspicion - now widely held - that the German establishment is not telling the truth.

The German public-service broadcaster ZDF did not mention the incidents in Cologne in its broadcast until last Tuesday, four days after the attacks.

The broadcaster has now admitted it was a "clear misjudgement" and says that since then, it has been "over-whelmed with hate and anger".

In parts of social media the idea of a "lying press" has taken root.

Some German papers are quoting police sources saying they are under orders not to report crimes involving refugees.