Likely, not many
I've read it. Much of the popular narrative isn't accurate. They had friends, there was a 'trench coat mafia' but Harris and Klebold weren't part of it. Their rampage was supposed to be done primarily with bombs, not the firearms they brought (the intent for those seems to have been to defend themselves while they laid the bombs then suicide, possibly by cop). Harris was a military kid who had a kind of tough, isolated upbringing and became truly sociopathic. Klebold was a normal suburbanite but had some serious depression problems and latched on to Harris.
I can never decide if I think that the presence of SSRIs in the systems of these shooters is a cause or if people with the disposition to do it just end up on pharmaceuticals. I do like the 'some asshole' idea. The media had made too many of these people famous. The Columbine killers themselves were inspired in part by Timothy McVeigh.