DCount counts the number of participants fulfilling a filter (first parameter):
DCount("[Finished]") - returns the number of finishers
DCount("T101>0 AND [Contest]=1") - returns the number of participants registered in Contest 1, who have a result in Result 101
If your filter contains text values, you need to escape the quotes with more quotes. Examples:
DCount("[Gender]=""f""") - Returns the number of female participants
DCount("[Gender]=""" & [Gender] & """") - Returns the number of participants who are of the same gender as the considered participant.
Note that the aggregation functions are not very fast and should be used rarely.