what you need to bear in mind is that your cornea gets its oxygen supply from the air - so if you cover it up with a lens for too long, cells will die and become opaque. If you are using soft lenses, these are quite porous so you can get away with the odd 24-hour spell, but you are risking damage to your cornea and eye infections. The old rigid plastic and even earlier, glass, lenses really let you know if you left them in too long. You can get lenses that are designed to be worn 24 hours, but personally I don't trust them. Sounds like a great way to get an eye infection to me.
As Clanad says, many people wear just the one lens. It depends on your own eyes, but if you are just slightly short-sighted then you could probably get along fine with just the one.