Hey Aswini -
At least with the java client, I don't believe that you actually have to download all of the ads. Instead, set up your selector, and then do something like:
Paging paging = new Paging();
paging.setNumberResults(1); //make this as small as possible since we don't really want to retrieve the ads
paging.setStartIndex(0);
selector.setPaging(paging);
AdGroupAdPage page = getAdGroupAdService(acctId).get( selector );
int numberOfAds = page.getTotalNumEntries();
I haven't used this for ads, but I use this technique for other objects and it works great.
HTH -
mm