PItalics are original; the bolding is mine:
I’ve pointed out that we don’t have the Q source. Since we don’t have it, you might expect that scholars would be fairly cautious in what they say about it. But nothing is further from the truth. Books on Q have become a veritable cottage industry in the field. . . . Not bad for a nonexistent source!
. . . .
Let me repeat: Q is a source that we don’t have.
(Bart Ehrman, Jesus: Apocalyptic Prophet of the New Millennium, 1999. pp132, 133)
That was when Ehrman was responding to scholars who use Q as evidence to counter his argument that Jesus was an apocalyptic prophet. (There are other arguments in the literature that contradict the apocalyptic prophet view of Jesus that Ehrman overlooked entirely.)
Then later,
With respect to Jesus, we have numerous, independent accounts of his life in the sources lying behind the Gospels (and the writings of Paul) — sources that originated in Jesus’ native tongue Aramaic and that can be dated to within just a year or two of his life (before the religion moved to convert pagans in droves). Historical sources like that are is [sic] pretty astounding for an ancient figure of any kind.
(Bart Ehrman, Did Jesus Exist? HuffPost 03/20/2012)
Within a couple of decades of the traditional date of his death, we have numerous accounts of his life found in a broad geographical span. In addition to Mark, we have Q, M (which is possibly made of multiple sources), L (also possibly multiple sources), two or more passion narratives, a signs source, two discourse sources, the kernel (or original) Gospel behind the Gospel of Thomas, and possibly others.
(Bart Ehrman, Did Jesus Exist? HarperCollins, 2012. pp. 82 f)
To be fair, Ehrman does eventually qualify the last statement by stating that our “having” is an “inference” but that word nowhere appears in the HuffPost article.
And these are just the ones we know about, that we can reasonably infer from the scant literary remains that survive from the early years of the Christian church. No one knows how many there actually were. Luke says there were “many” of them, and he may well have been right.