Cat-tent Spotlight # 6 This means war.
Although having multiple cats could be beneficial to each of them, you have to consider some things.
Do you have room for extra litter boxes?
Jackson Galaxy recommends a separate litter box for each cat plus one. For example, if you have 2 cats, have three litter boxes. This is for sanitary reasons. If both litter boxes are dirty, there is an extra one for the cats to use until you clean them. Cats will not do their business in a dirty litter box. Also, consider how much time you can devote to properly cleaning each one. An open litter box in a private place like a bathroom is best. An enclosed one will only make them feel trapped, especially in the fact of a threat. They also feel safest relieving themselves when they are out of view.
Do you have time and the will to properly introduce them?
Cats need a getting to know each other period before they feel comfortable with each other or dogs. For example, my last cat Sarge got really upset the first time he saw another cat. However, he got comfortable enough. Again, Jackson Galaxy recommends separating them behind a door and gradually bringing them together.
If you don't put in this work, you are more likely have to cat spats like this one below.