Array formula to count number of runners in an event

Hey guys I am looking to get an array formual that can simply be dragged down the entire page counting the following data (like in f19:f24), I was thinking it would be possible to use column C to identify when a row is withing the same event. Every event has a Unique Column C i am sure this can be achieved using functions like match and index, but I really can not get this to work. Obviously I knwo the standard Count method and could code a macro to put these in place for each set of data, but this would have a slower calculation time than one array formula. If anyone could help me out it would be greatly appreciated. I have attached a document to help make the problem at hand clear.

Many thanks in advance Alan

Perfect thanks, reputation added