Maybe Trux can do a statistical analysis of 'retention rate' of competitors. For example,
#1 total number of different athletes who have ever competed in a competition (2000-2011)
#2 total number of athletes that are still actively competing (i.e. one competition in last 12 months)
Yes, you can find it all at Apnea.cz here in the growth chart:
APNEA.cz ranking
As you can see the total number of competitors in the database (after removing duplicates) is slightly above 5,000. The yearly retained number is growing, and slowly approachoes 2,000. The data for 2011 is reduced mainly because I am still missing results of many competitions that organizers did not sent -
APNEA.cz ranking - missing competitions - 2011 (24 missing past competitions from 2011), and some more are still coming in the remaining next months. I believe that at the end of the year, the numbers will preserve the growth.
It is interesting to note that almost 1,000 (practically 1/5) of the competitors are French:
APNEA.cz ranking - France
Unfortuntely I miss results of French departmental and inter-club competitions, because it is impossible to find their results. The French competitor number could be 50% higer, or perhaps even more.
The numbers of Italian competitors would be quite high too, and at least double of their current counts, because they have I believe five or six age and performance categories for each gender, but I am adding results only from the top categories (from the Elite category, and sometimes from the 1st and 2nd ligues). So in reality, there would be almost 1000 Italian competitors too:
APNEA.cz ranking - Italy
With Spain it is even worse - I have only few of them in the database, but I believe the Spanish freedivers are numerous too. Same for Turkey - there are many CMAS competitors, but unfortunately I did not manage yet to get my hands on any results of their local competitions. It is similar for Nord African, Latin American, and Asian countries - I know there are also competitors (mostly CMAS), but finding the results is a challenge.
So in total, there may be more than 10,000 competitors who were active in the last decade, and over 2,000 in each of the last few years (possibly close to 3k recently).
However, as Goran tells, the most important is the member base, where you can recruit competitors. In the French clubs there are countless freedivers who perform much better than average competitors, and some of them even better than those at the top, but it is impossible to bring them to a competition. And many others do freediving just for fun, or as a physical or social activity, and have no intentions to compete. The usual ratio between competitors and non-competitors in clubs is 1:10