I have to research this as I don’t have first hand experience and my research indicates that it is normal, although rare, for a cat to have one kitten. Cats can have between one and nineteen (!) kittens in a litter (see below). The average is four. There may be two or more reasons for a ‘one kitten litter’ (1) as cats ovulate on mating the queen may have been mated only once and/or (2) she may not have had sufficient physical resources to carry a large litter to full term. It is possible, as far as I can tell, that the mother could have absorbed some of the embryos in order to ensure that the remaining kittens could be carried safely to term.
We don’t know the circumstances under which the person asked the question in the title. They may provide the answer to the question. For example, one person speculated that there was more than one kitten and perhaps the others were stillborn and not disclosed to the person who asked the question. A possibility.
A woman, Candice Grant, on Quora.com said that in her experience, over what appears to be many years, she has witnessed two queens giving birth to one kitten. In short, she is saying that it has been a rare occurrence for her.
Might there be certain issues with respect to raising a single kitten when there is normally a litter of four? Siblings either side of a fragile new-born while they are nursing for long hours is probably preferable in terms of keeping warm rather than being alone. The experts say that the kittening room should be at a temperature of 85°F (29°C) for the first seven days following delivery. You can see how warm it’s meant to be in those critical early weeks after birth. The room should be kept at around 80°F for the 3 to 4 weeks after the first seven days.
A single kitten is a special cat. Love her especially well.
P.S. The largest ever litter of domestic cats was 19 kittens. They were born on the 7 August 1970. The mother was a Burmese/Siamese mix belonging to V. Gane of Kingham, Oxfordshire, UK. Four of the kittens were stillborn – Guinness World Records.