If you use the definition of "mass shooting" used by Gun Violence Archives, this is the 115th mass shooting in the United States in 2019.
I have twice previously answered questions regarding statistics in mass shootings in On average in the US, is there a mass shooting 9 out of every 10 days? and Are handguns used in 80% of mass shootings?. For the sake of brevity, the conclusion I've reached in each question is that, due to disagreement as to how to actually define a mass shooting, people can change their definition to make the mass shooting statistics say whatever they want to.
In the case of this article, they are most likely using statistics from Gun Violence Archives. I've previously established that GVA uses one of the loosest definitions of mass shootings, which requires a combination of 4 people to be wounded or killed during a single incident.
This definition leaves out several of the requirements used by other agencies to define a mass shooting:
- no regards to fatalities, so a shooting that wounds 4 people and kills no one is considered a mass shooting
- whether the shooter counts among the 4 people, so a person who shoots 3 people and then is killed by police would count
- whether it takes place in one location or multiple location
- what the motivation of the shooting is, so domestic violence and gang violence are counted
Note that, while this is the loosest definition of a mass shooting, it is not necessarily wrong, as stated by experts quoted in this Politifact Article from 2017.
Gun control groups say it’s arbitrary to distinguish between a death and an injury. They point out a significant problem: Some shootings that injure a dozen or more people but don’t kill four people would not be considered a mass shooting under the more restrictive definition.
"I would submit that sometimes the only difference between a shooting and a murder could be a centimeter, an inch, an unlikely ricochet, whatever," Bueermann, who is now the president of the Police Foundation, which researches law enforcement practices, told the Post. "If we're trying to capture true gun violence in our country, a broader definition [of mass shooting] is probably more useful than a narrow one."
As to the exact claim made by the BBC, a cursory count of the number of rows on Gun Violence Archive as of 8 May 2019 shows that the Highlands Ranch shooting was indeed the 115th mass shooting in the United States in 2019. This is most likely their source for the count of mass shootings.
NOTE FOR COMMENTERS : There seems to be an argument in the comments about whether or not this definition is valid or about the tone of this answer. I am making no claims as to whether or not this definition is accurate or valid, just that the BBC is using this definition, and this definition is not agreed upon by everyone when counting the number of mass shootings.