Just that really.
I have two cats: sisters that are approximately two years old.
Should I add one or two more cats?
Edited to add: they came to the shelter in a group of four, with two other siblings. When they were adopted by me they were sharing a cage with two other (unrelated) cats. I also asked the shelter about whether I could add more cats in the future, and their advice was a male of similar age or two kittens.