I will own A LOT of bikes if I have all the money in the world. Realistically though, I think I'll cap myself at 4 - 1 mountain and 1 road bike and a backup for each in case anything happens.
Robin Williams owned A LOT of bikes, but all the money in the world won't make you happy, nor owning a thousand bikes, it surly didn't make Robin Williams happy, besides you can only ride one at a time, and personally I wouldn't want the decision as to which of the 1000 bikes I'm going to ride that day! I know, that's crazy talk.
One road and one MTB would cover it, not sure why you would need a backup bike for each discipline unless your racing, a MTB can be ridden on the street so that could be your backup too. It's not that I don't hear you, but if money is tight there really isn't any reason to have more than 2 bikes, paying off your mortgage or your car payment or your credit card(s) way ahead of time would be FAR more satisfying than having a couple of extra bikes. Again I'm talking crazy.