OK TM... I know the ladies here are trying to helpful, but let me tell you how it is...

You need a mountain bike for your off-roading - absolutely...

You need a bike for commuting, depending on how much stuff you need to carry... something sturdy with gearing and pedals for traffic and maybe pannier bags or something on it.

You need a light fast bike for road-racing, with race wheels, slick tyes etc stripped to the essentials only - I don't even have a puncture kit on mine (cause if you have a go at racing, you'll probably develop a taste for it)

If you want to do enduro rides, then you need a lightweight but strong bike, perhaps with a "bento box" and a bigger seat bag to carry your bits.

If you do TTs or triathalons, you'll want a bike set up with aero bars, aerodynamic rims etc, made for speed...

And of course, a general training bike... the one that has all the emergency stuff attached, the workhorse...

So I am guessing you need about 6 bikes... maybe only 4, depending on what type of riding you want to do...

Yours
Road (tongue-in-cheek) Raven

(BTW, I have 3 bikes at the moment - mountain, training, road race... my RR bike doubles as my TT bike, but am hoping to get a TT specific bike early next year)