Aluromaniac has great wisdom in her words. I especially agree that if you add a young or energetic feline to the works, add two.
And I have a litter of foster kittens who will be ready to adopt in mid-Dec at 13 weeks of age. I also have two young cats (6 months old), sisters who started out life feral. They’re very strongly bonded to one another, play with one another a lot (pestering the other cats less), and are warming up to me nicely. If I had to choose two cats to go with, in order of preference, it would be the two sisters, or two of the siblings from the foster kitten litter.
If you decided to add one more cat, it would be best to go with an easygoing, laid back cat, which is more likely to be an adult than a kitten.
One other trick to introducing another cat is, right before the other cat gets brought into the house, rearrange the furniture. (It breaks up territorial boundaries a bit.) Believe it or not, this trick of rearranging the furnishings to change up territory also works in fish tanks or aquariums (by moving around rocks, wood, or plants).