CGEL (The Cambridge Grammar of the English Language) discusses (at 3.3, p.349) situations where quantification is “expressed by means of a noun as head with an of PP as complement”. In these constructions terms like a lot, lots, a great deal, plenty, oodles and a number are in syntactic form the heads of the NPs in which they occur, but the semantic head is the ‘oblique’, the term which is the object of of:
a. Lots of workoblique is left to be done.
b. A lot of peopleoblique were present.
Note that in a lots is plural, but the entire NP takes a singular verb, is, because its oblique, work, is non-count, while in b a lot is singular but the entire NP takes a plural verb, were, because its oblique, people, is plural-only.
In these cases, says CGEL, “lot is number-transparent in that it allows the number of the oblique to percolate up to determine the number of the whole NP.”
In other words, semantics trumps syntax with these “number transparent quantificational nouns”: the verb agrees with the sense rather than the form.
In your sentence, consequently, the verb correctly takes the plural because its real subject is the oblique these 'discourse markers', not a very large number.
With many singular number-transparent nouns such as number, however, many people feel uncomfortable with a violation of syntactic concord. Consequently, you are just as likely to encounter a 'number-opaque' treatment. In the circumstances, neither can legitimately be labeled “incorrect”.
Note, by the way, that the phrase a very large number acting as a NP on its own, not quantifying an oblique, will take a singular verb:
A small number is expressed in words; a large number is expressed in digits; a very large number is expressed in exponential notation.