When do I need more than one bike? How many bike is enough? How many bikes is too many? I see questions where people want a bike to commute, road ride, and take on trails. 
Not one bike will do it all. 
When do I need more than one bike. 
How many bikes is enough?
If you have n bikes, n+1 bikes is the right amount of bikes to have. ;) 

Realistically, I think 2 or 3 is adequate - a cyclocross or non-racing road bike can do the first two tasks (road ride + commute) provided it has rack and fender mounts, and one mountain bike is likely good enough for the trails in one's area (if you go somewhere else where another type of mountain bike would do very well, say a fat bike in the snow versus your hardtail which is adequate for your area's trails or whatever,  rent it for the trip). A third would be a winter beater (an old rigid mountain bike, most likely) or something to take to the shady parts of town. 

Of course, enthusiasts can always justify more bikes (much like with lots of hobbies). 

[There is also the community of monster cross'ers, who build beefed up cross bikes which you can take on lighter trails, so if the trails are light enough, you could do it all with one bike!]

Too many would be when it starts affecting parts of your life which you're not okay with,e.g. too much money in bikes, not enough space, not feeding your kids for a new bike, etc.