using number of runnersfor a calculation

Hi,

I am not sure how to get the number of horses present in a race. I am developing a strategy which sets staking amount using a set percentage, and if there are 10 runners, the stake is 1/10th, if 5 runner, its 1/5th etc

Can anybody tell me how I can obtain the runners and be able to use this to set the Back/Lay amount.

Thanks

Mark.

amount = 1/runner_number