Learn English – “Not all people HAVE/HAS pigments in their eyes”

As J.R. says, singular or plural deserve will work equally well. The bare sentence may be parsed as either:

Only brave men deserve fair women.
Only a brave man deserves a fair woman.

There is, however, an overriding consideration. This line is a quotation from a poem by John Dryden, Alexander's Feast; or, the Power of Music.

'Twas at the royal feast for Persia won
    By Philip's warlike son—
  Aloft in awful state
  The godlike hero sate
    On his imperial throne;
  His valiant peers were placed around,
Their brows with roses and with myrtles bound
  (So should desert in arms be crown'd);
The lovely Thais by his side
Sate like a blooming Eastern bride
In flower of youth and beauty's pride:—
  Happy, happy, happy pair!
    None but the brave
    None but the brave
  None but the brave deserves the fair!

Dryden wrote it singular, with ‘the brave’ referring specifically to Alexander and ‘the fair’ referring specifically to Thais.

ADDED:
In present-day English we no longer use the ADJECTIVE to refer to a single person except in epithets (e.g. Alexander the Great), only for classes of people. The singular therefore sounds odd to anyone who does not know the source of the line—which is probably 99% of the people who use it.

Maybe there is a difference here between American and British usage.

In America, we would say "The number of people has increased." (Omitting all the irrelevant parts of the sentence.) Then it makes sense: "number" is the subject of the sentence, and "number" is singular. There may be a million people, but there is only one NUMBER of those people. Thus "the number has increased". A singular subject with a singular verb.

I don't know if the usage in the UK is different, or if this was a mistake in the original document.