The vast majority of Biblical scholars accept that at least the prophetic portions of Daniel were written after the events that they claim to foretell.
Is there a source that gives the numbers on this?
it would be necessary to provide solid proof that the text of the book of Daniel as we have it today existed prior to 300 BC. Do you have such evidence?
Yup!
For one, we can establish an absolute baseline for when Daniel was written. It cannot have been written later than the second century BC, since our earliest manuscripts of Daniel are from the late second century BC, and its in the Septuagint, which was completed around 130 BC.
According to here, the Daniel manuscript 4QDanC dates to “’the late second century’ B.C.”. So we know it cannot have been later than 200-150 BC.
(This isn’t just one manuscript from about this time either - also according to here, three other Daniel manuscripts – 1QDanB, 4QDanA, and 4QDanE, have similar dates, being from the “Hasmonean period”, which began about 150 BC. )
However, because its in the Septuagint, we can push it back to before 200 BC. For Daniel to make it into the Septuagint, it had to already have acceptance within the Jewish community, and so must have been being widely copied in 200 BC. Us finding that manuscript from around this date confirms this.
So our finds of copies of Daniel and works that include Daniel can get us back to before 200 BC. We’re still not quite before Alexander’s time, however.
But the internal contents take us to well before that. Internally, Daniel doesn’t look anything like something written after 400's BC. Its written in Hebrew and Aramaic, but all of the apocryphal documents that are definitely from around the second century BC have come down to us in Greek. Which, by the way, holds completely true with the apocryphal portions of Daniel that do come from this period – Bel and the Dragon comes down to us in Greek, as does Susanna and the Elders (which was a sort of prologue), as does The Prayer of Azariah and Song of the Three Holy Children.
And in the Aramaic section it contains terms that weren’t used past the fifth century BC. According to here, putting “the preposition Ie before a king's name” was a practice that did “not survive past the fifth century BC”. It also notes that “Four words in the Aramaic, which are apparently of Persian origin, are not attested to after the 5th century BC. These are אחשדרפן דתבר תיפת אזרא”.
So inside and out, everything says Daniel is from before the 300 BC. The document itself says this is when it was written (in the 500’s BC). The language is exactly like something written from that period – and very unlike something written later. The manuscript evidence makes a date after 200 BC impossible, and the text doesn’t look anything like the texts that actually do come from that period (including all three false parts of Daniel that were actualy written in the 200’s BC – which look exactly like we’d expect them to).