You will probably buy the chain rings for your bike with the crank set or the complete bike and maybe not have the choice of how many teeth you have on them. The thing to remember if you do have a choice is the bigger the amount of teeth the bigger the gear and less teeth for a smaller gear.

On a road bike your chain ring choice on a standard double crank set would normally be a 53 or a 52 tooth chain wheel on the outside and if you live in a hilly area you would have a 39 tooth on the inner chain rings, and if you are not going to do much climbing then a 42 tooth would be better and there wouldn’t be too much of a jump between the gears.

This is one of the problems of having a big difference between the sizes of chain rings; the jump between the gears can make it difficult to find just the right gears of the road conditions.

You could also buy a triple or a compact crank set for you chain rings, these would be 30/40/50 for the triple and a 36/50 for the compact, these you can change for different chain rings, but with these they will cover all your needs.

On a mountain bike you would have a triple crank set with chain rings of maybe 24, 26 or 28 teeth on the inside for steep hills or very muddy conditions, the middle chain ring would have 36 or 38 teeth and the outer chain ring would be 46, 48 or possibly 50 teeth for the downhill or fast flat sections.