Workout design - race pace sets

I'm just wondering if anyone can explain the trade-offs and considerations when deciding the number of repeats and amount of rest for a race pace set? If one is training for, say, a 200m race and want to do a set like: N x 50 on interval I how do you decide on appropriate N and I? How does the training effect differ if you do more repeats with more rest versus fewer repeats with shorter rest? Are there any guidelines relating the target race distance to an appropriate total distance in the set? Another way of phrasing the question is: if I determine that I can currently complete N repeats on a time interval I, as I get better how do I decide whether to do more repeats or to take less rest? Thanks, Lindsay