ls -l | wc -l is the most handy (I guess) for counting number of files when count is low
ls -f | wc -l is bit faster

But the ultimate I've found somewhere is:
find . -maxdepth 1 -type f | wc -l

It's amazingly faster than 'ls -f' and I believe with tuning maxdepth it shall be able to search in nested directories as well.