I am all for at least two.
I had 2 and had another in quarantine before I was to introduce them and the one on their own was very subdued compared to the other two. Infact I was so concerned by it I had to get another to keep it company (from the same litter). The change was incredible, really amazed me.
I am a BIG advocate of more than one.
Cons - only money and 2 wont cost more than one, it's when you get into massive numbers that it costs alot more.