Many cats are quite content to be solo cats even though they may love spending much of their time with their human family.
As we are such a sociable species, we can sometimes think cats get lonely when we are out at work or play.
Cats are originally solo creatures who lived as both predator and prey. When they do live in colonies, these tend to be groups of related females who are related and have grown up together. The number of cats living in these colonies tends to reflect how much food and shelter is available for them.
Some cats can live contentedly as solo cats in a home as long as they have all of their social, environmental, and physical needs met so that they feel safe, are both mentally and physically stimulated, and have a suitable and predictable home environment.
When we bring home an unrelated kitten or adult cat, this can be a shock and they can struggle to adjust to this perceived intruder. Likewise, the new cat may perceive the resident cat/s as being intruders in their new home.
However, cats can be very adaptable and sometimes they can form close relationships with the new cat or at least learn how to co-exist.