This depends entirely on the cats. For bonded cats, a multi-cat environment is absolutely fine. Bonded cats are often siblings which have been raised together, and are typically of a similar personality type.
Putting cats together and expecting them to bond, however, often does not result in a positive outcome. Cats are solitary hunters by nature, and although they may have some social interaction with other cats, they will have their own individual territories and will want control of their own resources, such as food, water and litter tray access.
Due to their nature, it can be very difficult to add a new cat into a household, as this will be seen by the already resident cat as an invasion of their territory. However, planning ahead and taking in cats together, or planning a structured and slow introduction, can result in good comradeship for cats, and can be beneficial for them.
It is also worth considering your local environment. In areas that are already densely cat populated, adding multiple other cats can cause a lot of territorial aggression, fighting and the risk of spreading infectious diseases – not all of which can be vaccinated against. Keeping multiple cats inside may solve the outdoor population problem, but is also much harder to manage, as cats will find it more difficult to escape an unwanted or stressful social situation.
Multi-cat households need careful planning to work. Adding a new cat into a household is a large undertaking and should be done after discussion with your vet.
For more information on introducing cats,