It does depend on what a person can handle, but in my experience AT LEAST four is ideal. I currently have three. Here is my reasoning: Jack is a snuggler, lazy guy all day, very mellow. Watson is ADD, hyper, up and crazy and running and jumping. So you see how the other is not exactly what they'd ideally want in a companion. Watson is always saying, "Hey, come play." and Jack is always saying, "No, I'm sleeping." Enter Cleo: also a snuggler. So now Jack has a perfect companion and Watson feels like a third wheel. Hope to rescue another with his type personality so he can have a play mate.
Just think of it in terms of friends. If you only have one other, where do you go when you have a fight? What if your personalities clash? Out of three others, you're likely to find an ideal companion.
You could get lucky and have two that love each other perfectly. In which case, sure have two.
The only things to be said for sure: don't have one sad lonely little guy and don't have so many you can't properly socialize or care for them.