This look at the matter of the grammatical plurality of single groups of plural items is well worth reading in full.
- Some do hold it must always be singular.
- Both can be found, from the middle of the 18th Century on.
- The plural is the more commonly used, has been for some time, and its relative popularity is growing.
- Respected writers who use the plural include Charles Batteux, William Hazlitt, John Keats, William Makepeace Thakeray and Mark Twain. (John Keats' was once considered a grammatical rebel for such things as the double be in "...how diligent I have been, & am being", but I doubt many would even notice why that was considered bad grammar by some in the nineteenth century. Mark Twain made heavy use of non-standard forms, but that was a conscious use and he was not doing so in the example).
I'll also note that John Cowan (who often remembers such things) is quoted thus:
J.R.R. Tolkien once received a letter (addressed to "any Professor of English Language") asking him about the rectitude of "A large number of walls is/are being built", and saying that "big money" was riding on the issue. He answered, of course, that you can say what you like. His original reply is not in print AFAIK, but a letter to someone else referencing it is in The Letters of JRRT.
My own opinion, is that the singular is easily understood as attaching to the noun that describes the group, just as the plural is easily understood as attaching to the individual members of the group. Neither clash horribly for most readers, though some do hold strong opinions.
My own use doesn't even side firmly with one or the other (my preferences here are given not as prescriptive decrees, but using myself as an example to demonstrate that usage may not even be firmly on one side or the other with a given individual).
If a phrase clearly refers to the individuals, I would strongly favour the plural:
The group of protesters were holding placards.
Some clearly to the group:
The group of protesters was twice the size as an hour ago.
Many could be interpreted either way:
The group of protesters was/were blocking the entrance.
In which I am likely to just mentally place in one of the first two categories, and write accordingly. Note that even with the first two, my strongly favouring one or the other does not mean I find other choices to be incorrect in others' writing.
From what I say above, you might expect that I would always favour the singular if the individuals aren't mentioned (even if I wouldn't "correct" another for doing otherwise):
There was a plethora.
The plethora of piñatas was/were gaily coloured.
But a group almost always at least suggests its members, and this question deals with one example of that, but there are others.
There are technical contexts where I would much more strongly suggest that expressions specific to the group or to the individuals be distinguished: The distinction between sets and their items in mathematics, or between collections and their contents in object oriented computer programming. Even here, I wouldn't so much say that a given form was wrong, just that there is a heavy benefit for precision and often you are working to help readers understand that very distinction, so an extra bit of prissiness may pay off.