Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMAR
realize that there is more than one subject here. (A sentence is not
limited to one.) Effectively, this sentence is saying two things: There
are some people who are late for everything, and Horatio is one of
them.
Albertine is the only one of the guests who needs a lift.
Again, there are two subjects here, but the focus is different. Effectively,
this sentence is saying that there is one guest who needs a
lift, and that one is Albertine. It is not saying that there are guests,
plural, who need lifts.
As the witnesses filed out, more than one was overcome by emotion.
Logically, more than one implies a plural, but by convention this
construction is treated as singular. (Note that you would refer to
more than one witness, not more than one witnesses.)
Included with each set are one or more evaluation forms.
One might argue that this is a case of alternative subjects linked by
or where the part closest to the verb is singular and hence the verb
should be singular. However, by convention, the phrase one orrrwre
is taken to be plural.
It is estimated that one in four adults has difficulty reading.
Although constructions such as this obviously refer to a plurality,
grammatically the subject is one, so the verb is singular. However,
many authorities say, with some reasonableness, that the plural form
should be considered acceptable as well. Which way you choose to
go may depend on the level of formality of your writing.
A number of students have already registered.
The phrase a number of is always treated as plural.
The number of cases was surprising.
The phrase the number of is always treated as singular.
UNUSUAL PLURALS AND SINGULARS
As discussed under "Plural Formations" on page 42, not all nouns
in English form their plurals by adding s. The majority of nonstandard
plurals do not cause confusion, but a few are regularly misused as
singulars. Conversely, some singular nouns are taken to be plurals.
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