If you want multiple cats, introduce them into a new home at the same time. That way no single cat can lay claim to being there first and cause territorial disputes. Unfortunately, this isn’t always possible so it’s likely you’ll have to introduce a new cat into a home where an existing cat already lives.
So how can you get a new cat to get along with the current cat (and vice versa)? Start slow. Let the cats know of the other’s existence but keep them separate for a while until both cats can get used to the other one.
Give both cats treats to help reduce their stress. Try swapping blankets so the other cat can smell the newcomer’s scent. Then pray a lot and hope everything works out, but introduce the cats gradually to increase the odds the cats may get along. Now the hard part is getting the cats to get along with you.