It's very easy to get obsessed with numbers and gear ratios. It's important for someone like me who plays with single and three speed bikes but with 24 gears, you should have a decent range, going from pretty low to pretty high. If you have at least one gear low enough to climb the steepest hills in your area and at least one high enough to pedal on the downhills if you're after out and out speed, and a few that provide a nice gear to cruise on the flat at a comfortable cadence, then it's all you really need to know.
The numbers you see being put about are usually in gear inches, which is basically a way of using the gear ratio and the wheel diameter to make it equivalent of the front wheel size if you were riding a penny-farthing.
If you want to know more, have a read of this -
http://www.sheldonbrown.com/gear-theory.html