A smaller number
. The comparative
It refers only to count nouns, such as marbles and people, not to mass nouns such as water or time: no English-speaker would say 'fewer water'.
There is a claim by some that it contrasts with 'less', but this is not on the same level as the previous. All English-speakers say 'less water' and almost all could and do say 'less marbles' and 'less people'. Nevertheless, some people who think they know what "correct grammar" is will tell you that you can't say 'less people' and that you have to say 'fewer marbles'. This is utterly unfounded, like all such nonsense.
Less in the sense of 'fewer' is sense 1.c. in the OED, and the earliest example quoted is from King Alfred's translation of Boethius (c. 888):
Swa mid læs worda swa mid ma
'So with less words, so with more'
In this worda is a partitive genitive and læs is an adverb, so it may be rendered 'less of words'. This is how the construction arose.
In 1481 Caxton wrote:
By cause he had so grete plente of men of hys own countre, he called the fewer and lasse to counseyll of the noble men of the Cyte.
In 1579 Lyly wrote in Euphues:
I thinke there are few Vniuersities that have lesse faultes than Oxford, many that have more.
The OED noted that this sense is 'Freq. found but generally regarded as incorrect'. Note that the OED simply records this fact; it does not endorse it.
However Fowler in Modern English Usage does not even evince awareness that it was popularly supposed to be incorrect. Under less, point 3, he notes that less and lesser were formerly widely used in various senses of smallness (think of 'the greater prey upon the less' and 'St James the Less'), but the "modern tendency" is to use more specific terms like smaller, fewer, and lower; "& to regard the now slightly archaic less in other senses as an affectation". He says "the general tendency is unmistakable".
Very well, that was in 1926, and it is clear that Fowler, who always attacked fetishes and pedantry, was not troubled by the fetish that you "have to" say 'fewer'. These days the general tendency is equally clear: the vast majority of us eat less eggs, strive for less worries, and perhaps write up less nodes than yesterday; and Fowler were he alive would certainly decry the insistence on 'fewer' today as a mere pedantic fetish.
There are constructions where 'less' is required with a count noun: 'one less thing to worry about' can't be said as 'one fewer thing'.